The use of a palm strike does not equate to a loss of strength or speed. A traditional fist may however enable greater effective range if thrown palm-up.
Perhaps one disadvantage of a typical half-fist palm strike (fingertips curled to top of palm) is that the increased surface area of the hand requires more room to navigate through an opening, and if the finger-edge side gets caught by limbs on the way to the target, the leverage exerted on the hand can be substantial.
One way around this is to maintain a traditional fist, but to cock it back slightly, enabling a palm strike in a more compact form. Depending on wrist flexibility, this may cause the first knuckles to lead the palm slightly, but insufficiently to render such a strike ineffective.
If the target of the uppercut is the bottom of the jaw/chin, a palm strike is much less likely to incur damage to the striker. The surface of the impact area is however greater, so the force will be spread over a greater area than with a fist.
The jawbone is sharp and strong, well-suited to slipping between the knuckles upon impact. In situations enabling an uppercut, the striker will more often than not be able to employ some form of palm strike rather than a fist. Given that a palm strike executed cleanly is extremely effective, it seems a safer uppercut option than a fist in most circumstances.