As long as you adjust the P&H formula
There can come a point where the number of province types is excessive, old school BR had just the one type. Bjorn's P&H allows 4 (I think), but with modifications for up to 4 features such as rivers, capitals, etc.
What I'd like is some sort of maximum cap sometimes, for example one of the terrain types I added to the P&H was intended to have a maximum source level which would prevent other terrain types increasing it, after some fun and games I came up with a way of making the P&H do what I wanted but it was very messy and would have been time consuming to implement in practice.
Otherwise I'd generally suggest that restraint in province design is the way to avoid multiple terrain types - if the DM really wants to have 4 terrains, then presumably they have a reason (guilty as charged in some cases), mostly from a quick flick through the P&H I only have 1 or 2 with at most a handful of 3's - many of which are quasi-2's as they are low mount + med mount, or forest+ancient forest/wood or some such where the impact of the 3rd terrain would be minimal. I only saw one 4-terrain province (Monsedge) so in practice the need for a formal limit seems minimal. The number of features otoh is routinely 3 or 4 so if a cap was wanted to simplify the P&H that would be the one which had an impact - but I confess to liking the features, they let me add a population point when monsters, etc need it most
