Did Larian take out the "That action can be used only to take the Attack (one weapon attack only), Dash, Disengage, Hide, or Use an Object action" from Haste and even stricter limitation from Fast Hands? As for Bonus Action Spells, you should be able to cast them faster. For 2 Fireballs, you need Sorcerer 5, Quicken Spell, and you consume 2 of 5 Sorcery Points and both 3rd Level Spell Slots. For 3 Fireballs, you need Fighter 2/Sorcerer 6, Quicken Spell, and you consume Action Surge, 2 of 5 Sorcery Points, and all 3 3rd Level Spell Slots. It's very valuable, maybe OP, but it consumes resources. Anyway, Fighter 2/Sorcerer n isn't on the list of most OP multiclasses.
To report only my understanding of the facts and not express any smidge of opinion :P
- Correct: the Haste limitations of 5e aren't (currently) implemented in BG3. You get a full additional action.
- Correct: the Fast Hands limitations also aren't (currently) implemented in BG3. You get a full additional bonus action.
- IIRC Quicken was changed to cost 3 sorcery points. In exchange, the "if you cast a bonus action leveled spell, you can only cast a cantrip with your main action" restriction of 5e was removed in BG3.
- The Fighter 2/Sorcerer X is strictly more powerful in BG3 than in 5e due to the above (among other changes). However, it's unclear how it compares to other multiclass combos in BG3, at least partly due to us not having all the information about spellcasting for multiclassed characters.
At the very least, it will be interesting to see what interesting and/or OP multiclass combos are found in BG3. The typical 5e OP-multiclasses are well known at this point. But even if we knew the full BG3 multiclassing rules, theorycrafting isn't necessarily a perfect replacement for actual experience. I imagine some combos will only become apparent after they're discovered in game and posted about.
Especially when you take into account the plethora of as-of-yet-unknown magic items that will be in BG3.
