So, I haven't tried the exercise myself, so maybe it's easier than I've been lead to believe; but I keep hearing that the problem is that those "small, factored tools" talk to each other and pretty much only each other and that it's very difficult to, eg, rip out and replace just one of them. The argument, as I understand it, is that SystemD is "modular" in theory, but is a monolith in all practical terms.

Note that I'm not arguing for this position so much as restating it to the extent that I understand it. I'm currently using SystemD and while I have my doubts about it, it doesn't cause me any specific pain ATM, so I haven't bothered messing with it.