Invisibility is what makes boss ending alpha strikes easy.
Invisibility + Haste + Dip/Poison + Action Surge = dead Hag without even needing Giant Strength. The way BG3 gives you a full surprise round after you attack them from invisibility, you get something like 17 attacks before they get to do anything. It doesn't matter how many exactly since it's more than enough to kill bosses. The surprise round needs to be skipped entirely. The initial attack should BE the surprise.
Is there a surprise round in 5e?
The element of surprise has always been a useful feature in D&D, but the Surprise Round, a free round of actions that the party gets before combat that had been a staple since Third Edition, is nowhere to be found in Fifth Edition.
I think there is a surprised condition in 5e, but it involves rolling Stealth vs. Perception. And for sure anyone doesn't get two turns to act freely like they do in BG3.
Larian should go with the skill contest when attack intent is declared. If you win both Perception and Initiative, the tables turn. That's how I would prefer my characters with high Perception to play rather than just sit there and take a beating. Stealthy Rogues also need to be better at surprise attacks than knights in Full Plate. It's silly you get to walk up to someone with a 100% success rate and whack them with a Greataxe and they "can't see it coming" no matter how good their senses are.
The whole thing just snowballs to insane levels with all the consumables and extra actions and weapon Bonus Attacks Larian decided to throw in. Like a dual wielding Thief with their extra BA using Wyvern Poison. It's just a mess. Even the hardest encounters can be so easily cheesed.