Yea, making a unit cloak is difficult with an arbiter. It is mainly luck, and it sometimes takes awhile, so I don't think it's worth it.
One unit cloaked is very good. But, floating a drone is easy, and it is effective early on in the game.
Arbiters are a nice unit, but like Skuzzy said, they take a long time to build, plus to get all the upgrades and all the buildings that takes a while and it takes a lot of money. Recall is nice, but what I like to do, is send a couple air units in for a distraction, then take a bunch of ground units and recall them over. But, I think the freezing technique is useless.
But, with all that said, arbiters are good units, I just wish they had a better attack.