I've put in quite a bit of time testing out all of the mercs, hugely inspired to find the best possible uses for them as dps, crowd control, and overall making them as effective as possible in all ways possible. Mercs are extremely powerful, most of them can completely carry you all the way to a5 destruction if geared right.
While figuring out how they work exactly, i've come to a few conclusions. When they have their Innate buffs like Heroic Strength (Freshly joined game, untouched inventory,) They are very hard to kill, they do some serious damage, almost playing the game for you. I don't feel this is a bad thing. If you need help somewhere, a merc could be a true stand-in for another player, and often a better one. I love that. Yet, as I read through some of the skills, they sometimes look really unnecessary
Ranger- Auto attack is slightly weak, Cascade pales in comparison to priestess' spore arrow
Priestess-Pagan Rites is crazy unnecessary. Her heroic strength makes it basically pointless
A2 mercs in a good place. All three can be extremely useful if built right.
A3- Bloodmage and Abjurer are amazing. Necrolyte is the weakest of them by a large Margin. I hate Charm, Hunting banshee is crap, his Miasma is weak,His Venomous Spirit is negligible. His Hailstorm is good, but with his AI, he doesn't cast it very well most of the time.
A5- In a whole different league. By far the strongest merc. Shower of rocks auto attack, massive aoe stun, and Ancients Hand absolutely wrecks bosses. Huge damage, best tank. His Warband skill is negligible though. Those barbs he summons seem like they have about 1hp and do no damage.
I know mercs will be probably no priority, but I think some small changes would be great, just to refresh their interest.