I'm almost sure they do work...map clicks too (if you are refering to OnMapSingleClick-dunno if there's anything else)...
I use fired EH on tanks all the time...for the enhanced shock dust.
I use fired EH on infantry to give them less accurate ammo.
Oh just remembered...
I run event handlers from init lines and they're checked at the start of the game...no hassle with declaring public variables...
...but there's bound to be someone here which can explain/clarify better than me