Flamestrike
Legend
I think the surprise rolls really show their weakness when a new combatant that nobody is aware of joins a battle a few rounds in. Let's say, for the sake of argument, an invisible and silent assassin joins a fight between Our Heroes (the pcs) and an oni and its ogre minions on round 3. I'm curious- how do y'all handle that? Insert a "surprise round" during which only the newcomer gets to act mid-combat? Just have it join the fight with no surprise? Something else?
EDITED to stipulate that the assassin is invisible and silent.
The Heroes are not surprised. They're literally in a fight with an Oni and its minions.
Surprised means 'caught with your pants down and unaware of any threats'. The heroes are aware of threats; their guard is up (thanks to the Oni and crew).
The invisible assassin gets advantage (invsible and hidden) but no auto-crit.