A daemonhost's power level depends on the daemon itself as well as the kind and level of bindings placed upon it. The stricter the control you place on the daemonhost the less of its power it can use. Cherubael was able to obliterate a warlord titan with a single blast when he was allowed fairly free reign, but his power level was greatly reduced when Eisenhorn placed a number of strict bindings on him (though he was still able to take out an enemy daemonhost who had lesser bindings, again showing that some daemonhosts are simply more powerful than others). As for who uses them, Necrosis is right, only radicals use them. Any grey knight or puritan inquisitor would simply kill anyone who uses daemonhosts.
All daemons are to purge, for they hide away from His light, mercy for them is chimera.
The use of daemonhosts is strictly forbidden by the ordo malleus, and anyone who uses them is marked hereticus, and hence an enemy to be destroyed, but for some the desire to control that sort of power is to overwhelming
Their powers obviously vary on their rank and which god they were alligned with prior to their capture, although that isn't represented in game, with them just having generic hosts, they have unique powers to them but they are very random, like a teleport and healing ability, the game rules are availible as a free pdf on GWs website if you want to see them
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