Can't speak for the whole systemd "opposition", especially since I'm not really a part of it, but I think it is rather a matter of multiple implementations sharing standardized interfaces, versus a single implementation with its custom interfaces: compare systemd's timers to cron implementations sharing crontab format, or systemd journal to syslog implementations, sharing its standardized protocols. With a single interface and different implementations, you can easily swap those, but with a monolith like systemd you are mostly stuck if the software actually makes use of it.
I think there are other reasons why people dislike systemd (and recalling a few articles with people ranting about it; I think there are many around), and usefulness of standards and alternatives/choice wouldn't necessarily make sense either, but here is at least one of the ways to view it.
As for init scripts, I think it can be even a bit nicer than that, since systemd provides some backward compatibility with sysV-style init scripts [1], but in that systemd acts almost like a swappable implementation.
[1] https://www.freedesktop.org/software/systemd/man/systemd-sys...