The older init systems were incredibly frail and often did not work. The reason is because Bash, as much as we all love it, is actually super mega ass. It's very bug-prone and actually one of the least portable languages ever invented, in practice. Because you can't, or shouldn't, call to any other binary from within Bash, otherwise you risk your portability. Now your script won't work the same on other platforms, or even the same platform with slightly different configuration.

But um... yeah... calling to other binaries is the entire ethos of Bash. It's a shell, that's supposed to be all it does. Even getting a "true" value should be a call to another binary.

Unit files, despite being a bespoke format for one singular program, are actually more portable. Like, WAY more portable.