You can fly any type of aircraft with a MPL as long as you have the type rating for it. After 1500hrs you could get the standard ATPL (not 100% sure if you would get a multicrew only restriction on it, but no large airline operates single crew anyway). Quality of training depends on the FTO. You'll notice the benefits of your 120 hrs of multicrew sim time during your TR, you'll have no problems passing that. Disadvantage of a MPL is that it is a worthless piece of paper if your sponsor airline can't provide you with a TR and LT.