It's not underrated. The thing is, you probably dont understand why.
The ball flight isnt any "worse" on a forehand slice than on a backhand. That's not the reason why people stay away from the forehand slice. In a nutshell, its this: 90% of the time you can hit a slice forehand you can hit a topspin forehand and the same cannot be said about the backhand side.
It's quite easy to junk ball someone with forehand slices, but chances are, if that's securing you wins, you could have done the same thing with a topspin forehand.
Eventually, you'll go up against someone that it just wont work. Slices have to be low, very low, with loads of spin to keep them low after the bounce. If you can hit a proper slice, you should have the ability to hit a proper forehand and backhand (at level).
I thought id never say this... but moonballing is probably better than a forehand slice up until high level 3.5 or 4.0 simply because its easier to hit and harder to punish. A bad slice is simply going to be a floater at the net or a ball that will bounce and sit up right in the strike zone relatively close to the net.
Overused and misunderstood? Yes.
"In the 1980's two men dominated--sometimes each other, most of the time everyone else."