True trials tires are great. There is a difference between cheap trials tread tires and true trials soft gummy rubber compound tires. They are meant to be run at low pressures, I run 8psi, and conform to pull traction like you wouldn't believe. And they last for a very long time. I raced with an IRC on the back of my 250F in tight woods HS and enduros for an entire season and it performed as well or better than a knobby. It is still at least at 85%.
Trials tires are not good:
In deep mud. They have close knob spacing and pack up.
On asphalt. They don't like high speeds, will get hot and shed knobs.
Flat tracking. Too much sidewall flex.
Brake sliding. Once they let go, they aren't as predictable as a knobby.
I have found they do fine in sand.
If you have a smooth riding style they are really hard to beat in 1st to 3rd gear tight technical stuff.