I think adventurers are very good province defenders. By far the best value for money. The only downside is that sometimes they just decide to leave. The other downside is that you can only get adventurers through contracts. Of course, at the beginning you can easily get by without any province guards. That changes once you "make contact" with other players and with the damned inquisitors.

Adventurers are good for the small maps, early in the campaign. Later on, though, even the largest groups of adventurers are easily pushed aside by AI heroes. (I'd say possibly in "medium" size maps, and definitely in "big" or "large".)

What is this "largest groups of adventurers" you're talking about? I only ever found one size of adventurers contract, and it totals 16 units.

No, that's what I mean. I guess sometimes it seems that I get smaller groups of adventurers when I hire them. I could be wrong--maybe I'm confusing the adventurer guards that I fight with those that I hire (and those that are randomly around in locations). Point is that on larger maps, the enemies will have enough space to build up large enough armies that they will be able to handle the 16 adventurers fairly easily.