Well, Dispels remove negative buffs from allies... ^^ So they also hurt debuffers and summoners (of course)
Ohh, and the amount of buffs removed only changes the duration of the Fire Elemental, not the power

Power is done via skill level. And the Elemental ist strong enough to take out a whole army, it is more like some support.
Same for Treant, just that the amount of faeries changes, power via level. But since they deal magic damage, most buffs won't make them more powerful. Just like the Elemental, they are too weak themselves.
AC Dispel: No additional effect on summons, on other units it is ruinatious, -18 armor on level 6... together with units that is powerful, but AC should be able to kill them by herself, too... think of Gale and Tangleweed.
TC Dispel: Power via skill level, Duration of the Blade via amount of buffs/units killed/removed, slow duration set... well, not much to say about it. It is fun to catch a (de+)buffing gravel with this, Ebb and Degen on Level 6 with mass int/evoc damage... GG Grav. He is slow anyways and double-slowed, he will get owned by Degen.
-Ham