So far we have only seen preta path being used by karma but there may be others who use the seals such as the inner or other Otsutsuki. Im wondering if one karma seal represents one of the six paths.
It may be like a partial rinnegan until it levels up. Momoshiki may have gained the palm rinnegan from absorbing nature chakra with karma.It's possible considering Body type Rinnegan's exist.
It depends on how good you are with sage mode. Karma has a transformation on top of buffing base stats.The power of karma, an analogue of the chakra, the Beast of Power - everything is fine, but not Genjutsu, and the sage mode is better.