It's faithful enough to 5e that my partner and I broke out the players handbook to do some long term class planning together. A couple of things are different, like buffs to frenzy barbarian and changes to roleplay feats or spells to have a more mechanical benefit.

But yes, as a long term DM for 5e, it's faithful to 5e.

Larian has been absolutely phenomenal through their process on both of these. Kept with the 'it'll release when it's ready' model, the exception with the alpha/early release on BG3 which I would say helped improve the quality of the Release product that much more, through testing/reports and cash influx without the 'pre-order today, get whatever you get tomorrow' mantra.

The most significant change I noticed was you can cast any number of leveled spells per turn. That’s a pretty significant shift from 5e’s rule of only one leveled spell (excluding using action surge if you dip into fighter) per turn.

However it makes the player stronger so I doubt anyone is really complaining about it.