Here are a few possible drawbacks in having a Sea Hare.
#1 When they get harmed they can release a dye like substance similar to an octopus. So if something like a big Turbo snail or an Emerald crab push's a rock over (which as you all know happens quite often) and the rock lands on the Sea Hare it can crash your whole tank. The rock doesn’t need to be big but just large enough to make the Hair think it’s under attack and it will release the dye as a defense. The defense or death of a Hare may not have such bad consequences on a large tank. In a small tank it could have dire affects on your coral's and fish before you had time to see and correct the problem with a massive water change (hope you have it pre-mixed). It could happen at night or while at work.
#2 When you look at a Sea Hare you can tell its just one big mass of flesh. When that flesh happens to die behind your rock or up inside your pile of live rock, where you can’t see it or access it, ouch. Then you will wonder why your ammonia keeps spiking even after multiple water changes. Say a fish dies, well normally the hermits, snails, worms, and crabs eat the dead flesh as fast as it decays, they like the soft spots first (decaying flesh = start of bad nutrients in water). The problem with the Sea Hare is its fat, very fat (though it does stretch itself very thin). The clean up crew may not be able to keep up with eating all the decaying flesh.
So basically the Sea Hare has good and bad qualities. I like them in my fish only tanks, I just move them from tank to tank as needed. I don’t have much L.R. in them and there's no place to hide if they happen to die. I don’t have them in the reef tanks.
There are reasons for having hair algae, solve the problem of to much nutrients in the water which feeds the algae and you won’t have to worry about rolling the dice on something that could potentially crash your tank.
Good luck