The majority of mods will be on Nexus at first anyway so there is no reason not to use that platform. It's really pretty easy as long as you have Vortex - but even still you can install them manually if you don't like Vortex.
There's plenty of reasons to not use that platform:
I don't like its interface. I don't like its organization. I don't like their Vortex client and I don't like having to keep up with mods manually to have them up to date, either.
Steam Workshop support would spare me from every single one of these things, so I'll gladly use it given the option.
And if some modders don't want to support it... Well, that's up to their choice, but they better be sure their mods will be particularly alluring, because otherwise chances are I simply won't use them.