If systemd has no problems and all the problems are with stuff it relies on breaking randomly because /usr is separate, that's fine. I thought systemd was explicitly relying itself on stuff in /usr, which for an init(8) replacement of all things seemed terribly, terribly unwise. If there's anything which has to work when the system is not fully up, it's init(8)!
(As for why separate /usr is necessary, I've had several reasons in the past. Lots of diskless systems with fast networking and very little storage is one of them: / on the limited available storage speeds things up a *lot* because /lib is local, but there's nowhere near enough room for /usr to go on there, it's huge. Not putting all your eggs in one basket is another reason: more than once I've had FS damage blow away /usr, but / and things like /etc were unaffected so reconstruction was easy. If /usr and / had been integrated, I'd have been visiting backup land.)