I thought I’d add my two cents - untainted (haven’t read the rest).
I never really used dragons throughout the game. It wasn’t that they were too expensive but more that I was worried about the lack of synergy between them and ice mages. A lot of levels have their best choke points right next to Azra, and between ice mages and dragons I’d rather have the slow than the (relatively low per cost) damage.
A lot of levels, too, it made no sense to spread out my units (mostly due to wanting to overlap healers). I couldn’t sacrifice both a healer and a dragon just to get a few early whacks in.
On the endless bonus, I tried using dragons as last minute defenders (I put most of my units near the top, and the dragons next to Azra). It was completely ineffective then as well. For most of the endless bonus the units at the top handled everything just fine. It only started falling apart when my units would die in one or two hits due to attacking mobs, at which point those mobs also breezed by the dragons.