To those making the basic and tired argument: Its a video game, in translation stuff has to be changed from the table top.
This is a lazy argument, because A) its very non-specific, and a free pass to not even try. B) Solasta exists, which is a MUCH more faithful version of 5E in a video game, showing it is in fact POSSIBLE to implement the rules much more faithfully and to good effect than has been done here so far. So please retire the argument.
For backstab and advantage: Per the 5E rules a rogue gets backstab if a friendly is within 5' of the target, no need to get behind the creature being attacked. Second, flanking is an optional rule, so its valid, but its also one of the more contentious and less used rules from community feedback because it frankly makes getting advantage way too easy.
Surfaces: They do exist in 5E, but never so easy, so long, or to such massive effect (at least at these levels). Many higher level spells can create a surface, but often they don't last long, or they require concentration which means they can be interrupted, and the opportunity cost of being what a spellcaster is concentrating on.
Rests: As currently implemented, short rest and long rest are effectively backwards from 5E. Maybe that's for playtesting (benefit of the doubt here) but as is, its strange and would be better if it were more like 5E.
Rogue ability farmed out to everyone: Cunning action is a big deal for Rogues, and contributes much to that class's uniqueness of playstyle and it really shouldn't be handed out to everyone.
Action economy: Currently, disengage, and several bonus actions are just way off from 5E, and there is no reason for it. This is a problem that potentially may make things worse, balance wise, at later levels as classes get even more abilities.
5E isn't a perfect system, but it has had the benefit of years of playtesting, discussion, errata, and balance changes. There is no reason, other than to make more work for themselves, for Larian to change things overly much, and with worse outcomes (see the issues presented in this thread and others).
There is alot that is very good in BG3, and it is my strong opinion that it will only get better. I think a big part of how it will do that, mechanically, is by sticking close rather than straying and taking liberties with the 5E system. If they didn't want to try to make it as faithfully as possible, why go through the hassle of securing the license and ruleset in the first place.