It doesn't matter, it's a long tail thing: on average user spinlocks can work, and even appear to be beneficial on benchmarks (for many reasons, Andy alludes to some above). But if you have enough users, some of them will experience the apocalyptic long tail, no matter what you do: that's why user spinlocks are unacceptable. RSEQ is the first real answer for this, but it's still not a guarantee: it is not possible to disable SCHED_OTHER preemption in userspace.

If I make something 1% faster on average, but now a random 0.000001% of its users see a ten-second stall every day, I lose.

It is tempting to think about it as a latency/throughput tradeoff. But it isn't that simple, the unbounded thrashing can be more like a crash in terms of impact to the system.