Nowadays not too much. For a while flymake languished while flycheck got faster, more, robust, added languages, etc. Probably just a function of when op started using Emacs
Note that "for a while" is in Emacs-time, so we're talking ~2013 to ~2019, i.e. longer than some entire popular editors, that flycheck was the normal way for someone using non-prerelease-Emacs.
(And even now I think I'd stick with flycheck if it weren't also for eglot using flymake by default.)
(And even now I think I'd stick with flycheck if it weren't also for eglot using flymake by default.)