It’s slow and heavy, and it does too many things. It’s a monolithic piece of code so big it’s getting too difficult to maintain, so it has more vulnerabilities than other alternatives. It’s also taking over the whole system, to the point where Linux systems will soon be Systemd/Linux instead of GNU/Linux.
It’s also developed and funded mainly by Microsoft, which is also something people don’t really like. Microsoft are trying to make it similar to Windows in some ways, which makes it way more difficult to debug random errors.
And it doesn’t follow the UNIX guidelines, which is just the cherry on top.