i had to join in on this discussion though I'm a n00b as most would call me.
to me it seems that the dangers of the eight gate chakra manipulation goes into effect when "gai/lee" releases the chakra itself in the different gates (whether lotus peacock or tiger.) i do believe gai knows and most likely has opened all eight gates but hasn't released the energy associated with the eight gate itself. basically it could be like pouring out your essence (just because you open a bottle does not necessarily mean its contents will spill out and the gate itself is a cap.)
once again its just my outlook on the gated seeing as the after effects become more strenuous on the user the higher it goes and the eight may be the users actual life force in the attack (someone speculated its name to be like midnight tortoise.)
logically speaking the gate being opened is opening the user the potential to that form of energy (yes there will be strain from opening any of the gates but that's where muscle memory comes in.) i believe its the usage of the technique to release said energy that makes the difference. once again just my personal opinion to this lovely debate.