Is PWM control for LEDs bad?
PWM brightness control enables dimming of white LEDs without significantly changing their color temperature or CRI.  It is however less efficient, and if done at lower frequencies, it can cause eye strain.  PWM is also unsuitable for many video or photography applications.  Current adjustment is more efficient and does not cause eye strain, but is more complex to implement and can cause the color to change at low brightness when used with white diodes.
Individual LEDs have an essentially unlimited number of switching cycles, so PWM itself will not reduce their lifespan.  PWM drives LEDs less efficiently, so they will run somewhat hotter, which could negatively impact their lifespan, although if you are dimming them anyway they will likely be running well below dangerous temperatures.
Depending on your application, you may consider PWM good or bad.