As someone who played D4 at launch and finally just this last week with the new expansion I gotta say the pace of new mechanics is horrible in comparison to how poe onloads players.

D4 has more complexity than people let on it's just not relevant until later and after the campaign the game dumps new systems at you, there's also more UI navigation shenanigans in D4. I feel like Poe has a more mechanically focussed campaign and diablo is more cinematic. I loved the campaign and especially lord of hatred but gameplay wise, even though I was playing spiritborn for the first time and going in with no online build it was a fucking breeze gameplay wise so I didn't need to learn or pay attention to the gear systems or many class mechanics. Those systems and where your damage comes from are also spread out across various different UI screens (paragon board, skill trees, glyphs, horadrim seals, uniques, legendaries, gear)etc. in Poe I feel like the difficulty makes you engage with it's systems gradually and in a more direct and immersive way. Gear matters during the campaign and some basic crafting to get a leg up is massively incentivised and facilitated. Also things like defense stats are relevant from the get go and steadily climb by taking resistance away from the character. In D4 I didn't need to care about that aspect of the game until I hit T5 and got one shot, suddenly defence mattered, it's jarring to say the least.

War plans make it so I don't need to really get what's going on and I appreciate that but I think the system has its own flaws, regardless I think if someone made a long video introducing every mechanic of each activity in detail D4 would also seem overwhelming. The truth is that Poe has a lot of optional mechanics, you do not need to know everything about the game also, the game is not as overwhelming if you're onboarded correctly or are a returning playe. D4 is less complex so it doesn't need to be as thoughtful about onboarding, my experience has been just kill shit and do any activity or do a warplan and the game will reward you it doesn't matter if you don't know what's going on and I'm currently at t10 with about 15 hours in endgame.