I can believe some people have implemented misconfigured thread pools, being worse off than with no thread pool at all. But this not an argument that justifies a "multitasking isn't a real choice for backends" position.
(Then there's bugs that happen in both asyncio and threaded programs, that stem from various queuing problems in the system, manifesting as work getting backed up in the concurrency layer. You end up needing backpressure in both worlds, thread pool limits are the wrong layer to do it.)