[drv]

I/O scheduler was probably a bad example, since you might not need/want one for fast NVMe devices anyway, but yes, they help ensure limited resources (storage device bandwidth or IOPS) get shared fairly between multiple users/processes, as well as potentially reordering requests to improve batching (this matters more on spinning disks with seek latency, since a strategy of delaying a little bit to sort requests could save more time on seeks than it would spend on the delay+CPU overhead).

The more general point is that if you need any of the many features of a general-purpose OS kernel, a full userspace driver may not be a very good fit, since you will end up reinventing a lot of wheels. Cases where it could be a good fit would be things like database backends or dedicated block storage appliances, situations where the OS would just get in the way and where it's viable to dedicate a whole storage device (or several) and a whole CPU (or several) to one task.