I would think that its unsafe since the turbo speed is only meant for a single (or fewer than the total) core running while the rest of the cores are sleeping.
That makes it unsafe from a lifespan perspective because when all cores are clocked at the max turbo-bin the temperature itself will be the highest.
This also means the CPU can/will exceed its rated TDP, so you need to have good cooling (better than a stock HSF).
But all the cores are already validated for operation at the max turbo bin, so functionality at the clockspeed is not the concern.