"Good but inconsistent" is really just another way of saying "not particularly good." The Premier League is full of players who look like world beaters on their day. Consistency is what separates the wheat from the chaff.
I have no problem with this deal since we need a big squad for next season but I do feel that Mirallas would be better off with a change of scenery.
[Separate Thought] I don't see how Mirallas is worth retaining but Deulofeu isn't. Both are inconsistent but only one has time on his side. I'm sure it comes down to Mirallas is willing to be a squad player, Deulofeu isn't, but still.