Soap holder or grab bar? If the latter, NOT a good idea. As you have already noted, they can't be trusted.
Smooth tile or pebble finish? Nothing sticks to those. For smooth finish, surface must be ultra- clean, and suction cup must be flexible, and moist. Rubbing a little vaseline into the cup sometimes helps, if it is porous material. Silicone caulk will hold it, but is a major pain to razor off when you want to take it down.