Certain "boss" enemies will not let my units attack them due to fear, but I noticed the Treants summoned by the druids don't suffer at all from this and quite effectively kills any boss. A low number of resummons is all it takes.

I'm just wondering if this was intended?

I felt it was a bit like an exploit but I can't compare to other races as I don't play them much and don't know their units.

On a sidenote, would it be possible to make druid summons restricted to every other turn or something similar. If only for personal use, would it be hard to do?

I'm fairly sure that all summons are impervious to fear, as they are not living.

Treants have the Elemental racial promotion, which makes them not be living units. I'm pretty sure all summons have either elemental, angel, demon or undead; hence none of them are alive. Fear is only supposed to affect living units (although there was a bug a couple of patches back that made only undead units (or those with courage, of course) immune to fear)