It’s a large and very complicated piece of software with a single implementation. It’s practically impossible to fork, so users are forced to adopt whatever changes the maintainers decide to implement. This could include things like forced dependencies (incompatible with mulb libc for example), or other poor design choices (like binary logging, which is very controversial). And it forces its adoption in places that do not want it (as in cases like the one we’re discussing here, where it’s becoming harder and harder for Gnome to be used without it).
I’m not going to argue about whether systemd is good software or not. But the biggest problem with it is that it’s basically a way for Red Hat to exert control over the entire Linux ecosystem.
Think of it like Chrome/Chromium. Everybody naïvely thought we were never be where we are today when it was announced, but look at where we are today. While it’s technically open source and an excellent browser, above all, it’s a tool for Google to exert its control over the WWW, such as disabling adblockers, implementing DRM, deciding which CSS/Javascript APIs should (or should not) be adopted, etc. systemd could very well be Red Hat’s vehicle for imposing similar requirements on desktop Linux.
As others have mentioned, this is a matter of threat model. To be realistic, a sufficiently determined government will always be able to access your communications, but companies like Facebook and Google can only access them if you give it to them willingly. On the other hand, if other people you communicate with do this by themselves, then you’ve gone through all that effort for nothing. It’s also worth pointing out that it cannot be proven that a regular phone does not have corporate spyware installed, so this may be another way your information could leak to companies.
That said, it is pretty insulting that tech companies have decided that they’re simply entitled to everyone’s private communication data. That for me is probably the biggest motivator in trying to avoid their services as much as possible.