NeXTStep/OpenStep had a great development environment and was full of innovation but even in the '90s it had old BSD components that were rarely updated and it really wasn't a great unix. Mac OS X has followed that pattern. Also Mach was inherently slow so running OpenStep on x86 hardware was slower than Linux or Windows - in Mac OS X they finally gave up on a pure microkernel and flattened the kernel to reduce the overhead of message passing through the BSD personality layer to Mach. But folks running OpenStep were running it for the RAD development tools and EOF that let you quickly design a UI with a very usable ORM that allowed you to take a desktop app and turn it into a webapp via WebObjects seamlessly. They complained about the *nix layer even then, but the unix layer was adequate and you could compile newer versions of tools you needed then as now.
After Apple bought NeXT, they upgraded the Mach component from 2.5 to 3.0 (from Apple’s MkLinux project). But it was always a hybrid kernel in both NEXTSTEP and macOS.
