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.

Does breaking your oath make you an Oathbreaker in BG3? Because that’s not how Oathbreaker works in 5e.

A Paladin who forsakes their oath would just be a fighter. Oathbreakers are specifically Paladins who call upon the forces of evil for their strength rather than the divine. They don’t just break their oath, but twist and pervert it for some dark power.