Pretty good gacha pull. You can appear 100% human, change form at will, resistant to most weapons. The main weakness to a slime is the lack of intelligence but that's not an issue for a reincarnated human, just ask Rimuru.
Slime is the symbol of flexibility and while generally regarded as a weak creature, there's countless stuff to be spun around them to make them strong.
Basically, slime is the perfect kind of creature for Japaneses to play around with.