Systemd is the most common software suite for managing Linux-based systems. It includes init, rc, device manager, temporary files manger, network name resolution service, cron alternative, machine manager, spoon, fork, nuclear reactor, remote control of the world, etc.
Let's see what problems could not be solved even after 14 years of development.
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Many maintainers use the entire systemd suite, even if they don't require all its components.
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Systemd-init, the core part of systemd, offers a wide range of features surpassing other init systems. More features lead to more bugs and security vulnerabilities.
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Source-based distributions may experience increased compilation time due to systemd.
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Systemd is exclusive to Linux and cannot be used on other Unix-like operating systems such as FreeBSD or OpenBSD. This limitation means that software dependent on systemd, like GNOME, won't be compatible with these non-Linux systems.
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The project is primarily led by Red Hat. There is a possibility of Red Hat stopping systemd development as a completely FOSS software and making it an exclusive feature of RHEL. Red Hat is a commercial company that will prioritize profit within legal boundaries. SystemD is still a tool used by Red Hat to increase its influence on GNU/Linux. The interdependence between SystemD and udev nowadays highlights the significance for Red Hat to encourage widespread adoption of their suite, rather than utilizing components developed by multiple teams of developers.
But you will still end up using SystemD, or at least some of its components. This is because opentmpfiles (the only alternative to systemd-tmpfiles) has been abandoned. Udev/Eudev has no alternatives and is a dependency for NetworkManager, Gnome, KDE, and many others. Gnome cannot function without logind/elogind. Systemd-boot is the default bootloader for certain Linux distributions (such as NixOS, for example). And it won't get better from here. The further we go, the more dependent we will become on SystemD. And this needs to be somehow stopped. Because the more widely used a software is, the more people will search for vulnerabilities in it. And with the amount of code in SystemD, finding a serious vulnerability is not that difficult.
The link lists 78 CVEs of varying severity levels opened over a period of 11 years. Many of them are patched. (I don't know how to easily check how many are patched. The NIST listings provide issue tracker links and severity levels, and the handful of CVEs I looked at had fixes released.) I'm not convinced this is evidence that systemd is unacceptably insecure.
I get that it's frustrating that systemd has such a broad scope, and that it's not portable. But these are trade-offs. In exchange we get power that we wouldn't get otherwise. For example tying device management and scheduled tasks into systemd lets us use the same declarative dependency management in those domains as in the init system. The system is able to bring up services only when needed, boot faster, use fewer resources. The non-portability allows use of, for example, Linux cgroups to cleanly shut down forked processes. Even if we were using an alternative like Upstart I'm gonna guess we would end up relying on cgroups.
Red Hat's role is certainly something to keep an eye on. But systemd is open source, and it can be forked if necessary.