In terms of how closely it resembles 5e, the current situation is that it barely does at all. There's so much homebrew that is so very prominent in the game that it overbears everything else, and large swathes of the actual 5e ruleset aren't implemented at all.
Basically; play the game for what it is, not what it claims to be; the less you come to it expecting it to feel like playing D&D, the more likely you are to enjoy it, because it simply does not feel like playing D&D at the moment.
There's been a lot of kick back about this, but thus far Larian have stuck to their guns about their opinion that 5e doesn't make for a good video game, and that their rules are better. It may change, it may not.
