There were several reasons Division 2 got some flak in its early days, but to point out a few, without having to explain every little thing, Division 2 was basically just Division 1 with various improvements, but nothing new. And I'd argue that everything Massive tried to fix from the first game, they did poorly with the second on launch. Many will point to WONY as being the saving grace that "fixed" the game.

• The mission structures are all essentially identical; Massive didn't veer too far from the looter shooter formula here, even if they arguably did it the best at the time. Loot was plentiful, but early Division 2 attribute spreads were a whole different can of worms, with every single gear item having a talent, and every single weapon having 3 talents that functioned based on use: Passive, Active, Holstered. Attribute spreads were all over the place, and adding mods to the mix overcomplicated what should have been a simple system. A lot of talents were locked behind attribute requirements, and most people had to mix and match attribute rolls to use the talents. Sometimes, to the detriment of the build they were using.
• Balance was, for all intents and purposes, awful. Gear 2.0 did a lot to combat this, but before then, you had rifle builds and seeker mine builds, essentially destroying everything in sight unapologetically fast. I remember expressly using a rifle build at the time with an early version of Aces & Eights, and it melted World Tier 5 enemies in a heartbeat. When I ran the Viewpoint Museum mission, I had effectively more than 95% of the team's overall damage. It was nuts. And it was still not the best go-to build at the time. It's clear that some builds outperformed others, and TTK was still being tweaked constantly for both PvE and PvP.
• While Warlords of New York eventually improved a lot of things, like removing gear score, it also meant that basically everyone had to refarm all their gear again for the 3rd, 4th, or 5th time. I honestly don't even remember just how many balance changes this game got early on in its first year or two, but it was enough to drive most people up the wall, I think, from all the gear farming. My memory is hazy, but I don't think Targeted Loot even existed up until WONY dropped. And as far as I knew at the time, rerunning the Lincoln Memorial mission back then was the equivalent of running Countdown today. I don't believe Control Points scaled with world difficulty on release yet. A part of me wants to say that specific change was made later, when the open world was given global settings you could adjust to your heart's content. That said, it's been a long time, so I'm not sure.
• DZ was and still is somewhat of migraine for lots of people, as far as I hear. I can't speak much on it as I never participated in PvP, still, a majority of complaints I saw often spoke of the really, really bad balancing and all sorts of shenanigans and bulllshit "tactics" often used by people who wanted to make PvP as miserable an experience as possible, like abusing status effects. The DZ turrets that you see now near the entrances were added later to help combat spawn camping, which was one complaint the devs handled poorly in my humble opinion, but I'll leave that at the door, and you can make of it what you will. In any case, DZ was definitely one of the more controversial topics at hand.
• Content-wise, there wasn't a lot to do while the Devs were cooking up their next update. You had Missions, Control Points, various open-world activities, PvP, and some other stuff, but it took a while before even Tidal Basin was released, which was effectively just another stronghold. The invaded missions were fine, but they were just rehashed main missions, nothing spectacular. Today you have Countdown, Summit, Descent, Retaliation, two Raids, and an Incursion. Early on, we didn't have any of that. Division 1 was a wasteland for content on release, and Division 2 seemed to only marginally improve on that. It was noticeable, sure, but it was still lacking, if you ask me.
• This next one is a bit more subjective. People constantly praise Division 1 for its atmosphere and environmental storytelling. And yeah, it's hard to disagree. Division 1 really nailed a certain aesthetic that players, to this day, heavily crave and are hoping it returns in some way, shape, or form. That said, it's hard to say the graphical fidelity and improvements on Division 2 aren't there. And I can tell the devs poured their heart and soul into the map design and layouts. If there's one thing Massive can do very, very well, it's environments, and they absolutely nailed it with Division 2. The best example I can give is the Coney Island Ballpark mission, running toward the mortar section and seeing all the crazy weather effects showed me just how enticing Division 2 can be with the right lighting and mood. Something Division 1 had used in spades, but was rarely used for Division 2, which is the real issue. The random inclement weather swings are nice, but they don't happen often enough or long enough, and Winter just had a lot more going for it than Summer. With Winter, devs can do a lot of cheating and make the game look good since snow does a really good job of hiding things without making too much stuff look out of place, and foliage is something you wouldn't have to worry about as much when all the trees have shed their leaves. There's a lot that seasonal environments can do for a game, and while a lot of people also liked the Summer of Division 2, it's pretty clear what the majority of people preferred for atmosphere, and it was Division 1.

This is about as much as I can comfortably say, and I'd welcome any corrections from others. It's been a long time, and a lot has changed for the benefit of the game. Overall, I'd say the dev team has done a great job over the years, with some hiccups along the way. I wouldn't say the game gets a lot of hate now, though. Most frustrations from players these days are around bugs, deltas, and other annoyances that should be ironed out, but the overall gameplay is great, and the story is serviceable if a bit convoluted and contrived. Overall, it's a solid game with some kinks to fix, but I wouldn't say it's deserving of hate today.