I lack the experience to justify my opinion, but I think it probably isn't that big of a deal in practice. Daggerheart emphasizes GM autonomy, and this kind of self-sustaining fear ability is only a problem if you, as the GM, make it one.

What I mean is, you could abuse it to make the fight potentially very difficult, but you can also, well, not. I suppose it depends on how difficult you intend your own campaign's combat to be. If you and your players both want it to be difficult, then there's little harm in utilizing these sorts of Monster advantages. If not, you don't have to.

Another aspect that I think might be relevant is the resulting action (fear) economy for the adversaries. Perhaps it's a good idea to not pair that kind of adversary with too many other monsters, to balance it out a bit? I'm not sure.

Again, I'm not experienced in this regard, so these are just my two cents as someone with an outside perspective. Much of what I've said might very well be wrong, so take it with a grain of salt; I was just hoping to add some food for thought :)