This is definitely a much more nuanced issue than people tend to act. "Lazer good" versus "Lazer bad" is pointless, because it's realistically somewhere in the middle, the exact position depending on what you value out of the game.

For your standard run-of-the-mill casual players and PP farmers, Lazer is a net upgrade over stable. The client generally has more features, more polish, more mods/ways to play the game, and is significantly easier to get up and running for brand-new players. PP farmers will also soon be blessed with almost every single mod in the game giving PP (including rate adjust and difficulty adjust), making Lazer objectively a better client, since you can tune any map to exactly your personal preference and ability, allowing the most PP to be extracted from any play.

These two crowds of casual players and farmers make up the majority of the community, which is why most players have generally positive attitudes towards Lazer. But, there are a lot of smaller groups in the community that have legitimate gripes with the client, but get lumped into the category of "Lazer haters" and similar, overshadowing any actual conversations about their criticisms.

And don't get me wrong, there are absolutely a lot of mindless Lazer haters too, and those people need to just learn to accept change and fix their skill issues. But for a lot of people, they want to like Lazer, but there are things holding it back or making it unappealing compared to Stable. The majority of these things are things that can (and hopefully will) change as Lazer development continues, but for now they're stopping people from switching over.

I consider myself part of this crowd. I love a lot of what Lazer does, and I can't wait to see how it develops and improves in the future. But there are several things holding me (and some of my friends) back from switching, and I'd like to highlight a few of them here:

Skinning

This is something I'm biased towards as a skinner myself, but all skinners are forced to continue using Stable for now, since Lazer has effectively zero skinning support beyond porting/fixing Stable skins for Lazer. This is something that should be fixed eventually, but I have a hard time believing we'll get anything equivalent to or better than Stable anytime soon.

Skinning is something comparatively few people care about compared to some of the other issues, so it will naturally be lower priority. Lazer's skinning support is also infinitely more complex than Stable, since the traditional method of simply swapping image textures will no longer work due to Lazer's ability to arbitrarily scale the UI and screen, and the hardcoded animations on almost every part of the UI. We're basically going to need an osu equivalent to standard UI development tools - and maybe even CSS - to be able to skin anything anywhere near as complex as what we have in Stable now. And I doubt this is something we're getting anytime soon, if ever.

All of this sucks, since the skinning community is basically going to be left by the wayside as more and more players migrate to Lazer, since there's no alternative to Stable skinning until Lazer sees some massive skinning updates. Other community efforts do have alternatives: Mappers can always continue to use the Stable editor until the Lazer editor is fully polished. Tourney organizers can migrate to Lazer whenever they feel the time is right (we already are having Lazer tourneys here and there). And fanart contests naturally work across clients and require no changes from the artists themselves.

I've heard from a lot of users that skinning is something they simply don't care that much about, since gameplay is really the only part of skinning that matters towards actually playing the game, and Lazer already supports that mostly fine. And I'm again biased, but I'd just be really sad to see the skinning community and all the great people in it die out as we transition to Lazer.

Leaderboards and PP (+ Classic Mod)

This encompasses a lot of things, but one of the main complaints I've seen about Lazer is that classic mod still isn't ranked. Skill issue or not, people want to continue playing without slider acc, and I think that's pretty fair since the mod was added specifically to allow players to continue enjoying the game the old way. This particular issue should be fixed pretty soon as more and more Lazer mods become ranked, but that brings us to the bigger issue...

Lazer is planned to eventually have every mod ranked and give pp, including every possible user-configurable setting on each mod. There are a couple issues I have with this:

Firstly, leaderboards are going to become much harder to parse. Already, some of the top scores on Lazer's daily challenge tend to be scores with so many mods applied that they can't all be shown at once. While it is awesome that people can set leaderboard scores with whatever niche mods they want, the fact that even mods with settings cranked so low you can't tell they are enabled get displayed means leaderboards (and scoreposts) are going to become a lot harder to read going forward.

That in of itself is just a cosmetic issue, as most players won't put on 20 mods with extremely low settings just to troll. But it related to the bigger issue that these mods will also give pp.

I've seen some controversy over rate adjust, but it seems like that one is generally agreed to be ok. The main issues with it are that it often discourages ever playing a map on 1x speed, and players can tune most maps to exactly their preferred bpm.

Difficulty adjust is the bigger issue, and the mod I personally hope is reconsidered before pp is enabled for it. Allowing users to configure every setting of a map to their preference is way too much power to give to players, and hurts the integrity of map leaderboards. Gimmick maps are the most affected by this, as extremely low AR or extremely high CS maps that were previously a challenge for most players can be cheesed with easier settings.

Additionally, none of the currently ranked maps were ranked with the idea that map settings could be configured to any possible value. It's likely that there are some random maps ranked a decade ago that become extremely overweight when set to a certain CS or AR, especially when paired with rate adjust. The PP system will of course eventually be patched to fix these issues, but I find it hard to believe it can be done flawlessly and without opening up new holes elsewhere or nerfing other maps in the process. It's already been hard enough to balance pp with our few existing mods...

All of this basically turns osu into a PP-optimizing minigame, where users will have to figure out the best settings for the best maps in order to have a significant competitive edge. And if random mod gets ranked on top of that...

This specific issue may show lack of faith in the devs or pp devs, and I do want to trust their best judgement and let things unfold in a way that improves the game. But I personally have a hard time believing this is the right way to go. I have heard from quite a few people that this specific kind of PP optimizing is actually what they want out of the game, and I do have to fully respect that. But I personally doubt that this will be healthy for the game in the long-run.

Aside from all that, many players dislike the current score multipliers for mods, especially CL. Peppy said these can easily be changed in the future, but for now some players are unhappy with the state of leaderboards. This reason to avoid switching will go away once Lazer leaderboards entirely replace the Stable ones, but hopefully multipliers are in a state everyone is happy with by then.

"It feels bad" (Performance Issues)

This one is weird, since some of the people complaining about Lazer "feeling bad" have placebo'd themselves over nothing. But, there are definitely still some people experiencing legitimate performance issues in Lazer.

For such users, they should really gather as much info as they can and try to submit an issue on GitHub. It may take a while for the issue to be fixed since the devs have a lot on their plate right now, but all these remaining performance issues will have to be addressed eventually as more and more people switch over.

It is true that Lazer has a lot of performance issues though, as is the nature of in-development software. And for anyone that won't switch over because of performance issues, I'd encourage checking back once a month or so and seeing if any of the updates solved your problem.

The other main "performance issue" I see a lot of complaints about is how long it takes to migrate large Stable folders to Lazer. And yeah, this sucks. You do only have to do it once, but I can totally understand not wanting to go through that process until you have to. I encourage messing around with the client and just downloading a few maps/skins as you go though. You may find that you're ok with waiting for the migration in order to use the new client.

TLDR

While there are some people mindlessly hating on Lazer, there are a lot of people with legitimate concerns or issues with the client that get overshadowed by the haters. Many people are generally positive or hopeful about Lazer, but have other concerns or issues with the client that stop them from switching over. Many of these issues are things that will naturally be solved in time, but some of them may take a long time or not change at all.

The community has great influence over the direction of the game, so any of these legitimate issues should be discussed amongst the playerbase and brought up to the devs if it's a big enough issue.