Thanks oosyrag for the suggestions.
I agree, though, I think "trigger once" is also checked every tick, it's just that it makes a much simpler boolean check, to see if it can reset. If you code your own equivalent trigger-once-style functionality I think it should perform about as well.
The "trigger once" condition is especially good for cases where you know the True/False frequency of the rest of that event's conditions will always be slower than 2 ticks. The event sheet's Nyquist frequency, so to speak.
So the case of on-hover is a bit tricky in that respect. If your buttons are really far apart you're probably safe. However, if you have a row of buttons close to each other, you can roll off one button and onto another button, without an intermediate tick where no button was hovered. And that can confuse a "trigger once" based system. It wouldn't if "trigger once" really was a trigger-type condition, but I don't think it is.
As far as I know, true "Triggered events" are implemented as message-subscriber style function calls internally, or something similar. The "On function call" is an example of a true triggered event, as it really doesn't check every tick. In fact, I think "On function call" may be an even more specialized version as the function name may be hashed or something to make it even faster than a normal true "Triggered event".