It had severe launch issues that was long ongoing even for 1-2 months. It's much better now though and they are still optimizing it with the latest row currently in testing, which I think may solve these issues for good.
The problem as I understand it was an architectural decision to remove the realms (regional groups of servers like USEast/West/EU/Asia) of Diablo 2 "Classic" so that everyone can play with each other. More fun that way, right? However, Diablo 2 has pretty lousy netcode (consider it's two decades old!) and turns out that one does not simply rewrite it to sync your heroes regularly to a centralized location as you played, turning out to cause a terrible bottleneck. Many such sync requests seem to have just straight out failed at peak hours, with characters losing hours of gameplay or even losing the characters altogether!
This is, again, much better now thanks to more or less emergency mitigations. But it took them a few months and it's of course a major goof up to not think this through better. By the time it was fixed, many had started losing interest in the game.
It's hard to extrapolate networking from testing to massive scales, I understand that, but these are network architects with years if not decades in the business of launching games and MMORPG's. _Someone_ should have been waving a little red flag here and told "hey, you do realize we're centralizing a core part of the game that was never planned to be centralized, and if these sync requests fail it's going to ruin their gameplay. So we're taking a risk and with very real fallout as well."
But maybe someone did... Maybe many in fact did. Such warnings may not be the end of the story unfortunately.