It is more than this. The patchers modify the startup including loading/modifying software in the kernel. And ,in most cases modifies SIP. So it does inherently reduce your security and, of course, you need to trust the developers.
"Safe"? As above.
Most likely "unsafe" thing is that something is not quite right and the system is unstable. This varies by hardware. Be prepared to go back to older supported macOS.
No.