I'm surprised to see nothing about it here (maybe BGG ?)
I think the reinforcement rule has a problem. Always reinforcing a monster is just not balanced. Imagine a map with Ettin and goblin. You may reinforce only one. It is obviously better to choose the Ettin. And it's the same for all monster. So, when choosing open groups, you will rather prefer big monster to small ones.
So, I think it's because of the rule. The "one monster" is the trouble. Monster cards should have a reinforce number, saying how much you will spawn for "one renfiorcement point". For exemple, only one Ettin, but 3 goblins. Because if not, using the kobolds are just useless. A runemaster blast attack, and everyone goes to the graveyard. Yepee, you may spawn back one by one, fanstastic !
If wonder if a house rule may be used. Here is my idea.
"the rule for reinforcement is revised. Each time it says "you may reinforce a monster", unless the monster is specified, you have instead a "reinforcement point". When using this point, you can reinforce half ot the groupe limit, rounded down (for a minimum of one")
What do you think of it ?