It shouldn't be, as there's an iPhone client, a Windows client, and you can use the web version for Linux. I have my complaints about OneNote, oh I've got complaints. But cross-platform isn't one of them. I don't have a personal notebook that use much anymore, but I've got one for a local non-profit that I work with, and a couple for work, all of which are accessible anywhere I have a connection.
That said, I just use vimwiki and Apple's Notes/Reminders apps. I've used OneNote since it was an internal Microsoft build running on my Toshiba Portege Tablet PC, but instead of adding features that have (IMO) been needed for over a decade, it looks like they'll be removing functionality in the next release. So it's vimwiki for anything remotely complicated, and Apple's apps for the day-to-day "remind me to buy cat food" stuff.
vimwiki, which means you better be using vim, isn't a text version of OneNote, but it's close enough. And in a lot of respects I find it less annoying than OneNote's quirks. Project management, daily log, work notes, things that could use bullets and mulit-level outlining and the like. Export to HTML if it needs to be printed. As there doesn't seem to be any kind of iOS client, I might write one of these weekends when I'm bored. But I dump the wiki files to iCloud, so I can get to it from work or home, and if I were desperate I suppose I could view the raw files on iOS.
Apple's stuff I use mainly in conjunction with Siri. "Hey, Siri, remind me at 8:00 tonight to pay the light bill". Quick, one-off stuff I don't want to forget and would like a reminder for later. No heavy, project-oriented stuff goes in there.