Spy definitely is at a huge disadvantage in this.
When they have level 3's engies can just sit up against a wall protected from trying to shoot them by their sentry, they either have to be really dumb/busy before they are in a position where you can stab and sap in 1 move. The only other way you can take them out is to sap there sentry first and give away you are a spy.
If they are using minis you have a slightly better chance, but you still have to face the choice of sapping first to give away position or going for the stab first, hoping you don't miss and you can sap the mini before it kills you.
Either way I am really not looking forward to facing the hoards of enigies, I might just give up on pubs during this event (class limit on tf2maps sever plz).