Conscious dentals (for the molar teeth) are very dangerous as there is an artery at the back of the mouth. Bunny only has to flinch and the artery can easily be caught = lots of bleeding
Rabbits can have their incisors burred conscious as many will sit still for it.
Doing dentals 'under a light g/a' can be dodgy too as when rabbits are gassed down with no premedication, they are very likely to hold their breath (anaesthetic gas smells)- this is obviously not good for them. Also, anaesthetic gas wears off pretty quickly so the vet will get so much filed, bunny starts to wake up so vet has to stop - give more gas and then resume filing/burring. This can actually make the anaesthetic longer (and therefore more dangerous) thn just giving a normal dose of g/a and getting on with it.