TL;DR: Making some developer decisions to creatively alter the D&D rules in order to make what they believe and hope will be a more fun video game is not some new, unique crime of Larian Studios. It is, in fact, core to the Baldur's Gate franchise legacy.
And that's fine. Many of the rule changes are more fun and towards a better game. The Ranger class, for instance, is changed for the better. Niche spells like Speak with Animals/Dead lasting until long rest or concentration is broken is good.
Many of the things people are complaining about are not because they are rules purists, but because some systems uses PHB rules, and other systems use Larian's modified rules, and that has a clear impact on the game's balance.
You get the Hit points, the magic spell slots, magical healing, and your base chance to hit and be hit from the PHB rules, but the surfaces have additional effects and damage, and chance to hit or be hit altered by effectively, +/- 4 to hit because of advantage to high-ground attacks and disadvantage to low ground attacks.
