It is possible, it just isn't quite as reliable as black powder piston launching. Black powder motors are essentially "instant on," and once they light they tend to come up to pressure without delay. For the most part, composites are the same way (especially faster burning propellants like Blue Thunder), but with White Lightning (D10, E15, etc), and perhaps BlackJack as well (if you want to piston-launch a 29mm model), the motors may chuff, letting out an initial bust of gasses before the motor has fully ignited. On a piston, this burst of gas is often enough to fill much, if not all of the piston volume, and in a worst-case scenario could kick the model off the pad and onto the ground, at which point the motor would probably light with the model aimed at an arbitrary direction on the ground, which would be bad. For this reason, composite-powered models that are launched from pistons are generally required to have a secondary guidance source on the pad (ie, a tower that could contain the model even if it happened to pop off the piston early). Though, I'd like to add that I think there should also be a requirement for a backstop as well--at NARAM this year, I witnessed at least one model chuff off it its piston, ascend most of the way up its tower, then slide back downwards again. Fortunately, the motor lit before it had slid back out the back of the tower, and the flight was fine, but it could have potentially been a problem if the model had dropped out of the tower before it did light.
But to sum all that up, yes, it is usually quite fine and very doable, especially with D21's and E30's. Be careful with D10's and E15's, but if you have a good igniter they are generally fine as well. You'll definitely want to be using a tower tall enough to contain the entire model at full piston extension + a fair bit extra (if you use a 20" long piston and have an elevated tower, a standard 3-4 foot tall tower ought to be enough, though 4+ would be best).