The only thing SPDK buys you is somewhat lower latency, which isn't that important for most applications because modern high-performance I/O schedulers usually are not that latency sensitive anyway.

The downside of SPDK is that it is unreasonably painful to use in most contexts. When it was introduced there were few options for doing high-performance storage I/O but a lot has changed since then. I know many people that have tested SPDK in storage engines, myself included, but none that decided the juice was worth the squeeze.