They are a HUGE gamble! The problem is each "type" of Sea Hare eats a very specific type of Algae which you may or may NOT have in your tank. Another problem is that they eat a LOT of it (require a LOT of HA for long term survival). Lastly is that they are only able to "Uptake" a small portion of the "Nutrients" they consume and they leave some good size "trophies" throughout your tank. I'd like to call them "Sea Mules" because they leave so many LARGE "Trophies" which can work to FEED future HA outbreaks!
Address the cause of the HA in conjunction with "controlling it" and you'll stand a much better chance of getting rid of it.
I've had more than my share of Sea Mules LOL