I don’t see any fundamental reason why systemd would be insecure. If anything, I would expect it to be less prone to security bugs than the conglomerations of shell scripts that used to be used for init systems.

The bloated argument seems to mostly come from people who don’t understand systemd init is a separate thing from all the other systemd components. You can use just the init part and not the rest if you want. Also, systemd performs way better than the old init systems anyway. I suspect many of the those complaining online didn’t really have first hand experience with the old init systems.

If a different init suits your needs better, then sure go with it. But for the vast majority of typical desktop/server stuff, systemd is probably the best option. That’s why most distributions use it.