I've been to both the Mucho Mas restaurants (my experience at Bogstadvein was better) they serve meat and claim a wide variety of vegetarian options. They do however use cheese which contains rennet which is not vegetarian.
At the Mucho Mas in Grunnerlokka we were assured only the enchiladas contained jarlsberg (or similar tine cheese) and that the burritos were made with feta not jarlsberg, alsas they arrived with some very melty jarlsberg. These were sent back only to find my mainly veggie boyfriend who had ordered a mushroom enchilada had actually received a chicken enchilada. Not so pleasant when you have just stopped eating meat!
After a few back and forths with the kitchen a veggie friendly meal (and very tasty one) arrived and we also recieved a discount for the mixup.
In general I probably would go back but I would be very specific about my dietary requirements from the start and opt for no cheese just to be safe.
Cons: They're not totally sure what is veggie