not so much uncontrolled pressure, but an unexpected pressure rise, boost creep is not like a wastegate that has lost its reference hose that would be an uncontrolled sharp rise to very high levels
Boost creep is a few psi more than intended, , for many this is dangerous to the motor as they may already be close to the knock or detonation limit
usually only happens in the low boost areas, once you get more boost happening the back pressures in the turbine and compressor reduce the level of creep
and in some circumstance boost creep can be handy , gives you a bit of a hp increase at the top end without so much mid range torque that may otherwise lift the front or blow off the tire
BUT it is much better to have a wastegate that can control boost properly and if you want to tailor the boost curves, use a good boost controller and get adjustable repeatable results