The most UNPOPULAR faction in Heroes 5.

Personally, I don't like the Sylvan faction and the Academy. The Sylvan have both weak abilities and weak troops. In none of the Heroes series has there been a situation where an upgraded unit has even one of its stats lower; and that's what we see here. Let's look at the HP of the Druids. As someone correctly pointed out, only the Archers and Ents somewhat soften the opinion about this faction.
As for the Academy, that's a separate conversation. The attack and defense of the heroes don't increase. I had a hard time beating the last mission of the campaign with a Wizard hero in the last game. I agree that the design of this faction is not very good.
;)