These are plain text files that I sync using a git repo on my private server. I don't need to travel much, but I do switch between 3 machines regularly, which means I have to sync them up often. I use git command line.
For anyone that already uses git for work, it doesn't seem like much of a difference from normal workflows.
I also use git to handling backup and syncing of a bunch of personal data.
One of my projects for the new year is I've started keeping a daily journal both for personal stuff and for work.
I have a git repo with two folders "Personal" and "Work". I run "vim `date -I`" to open a file for the current day and add to it periodically throughout the day. Each time I add something I just commit and push like I do already for work.
I've found that it has been helpful at work in particular for maintaining a running log of what I'm planning to do and what I've actually gotten around to.
Like all new years resolution things, time will tell if I actually keep it up.
But I guess circling back around: Git works really well for syncing things between machines. That's sort of what it's for after all.
Not a priority for me. I actually don't even own a smartphone. I'm not saying using git to manage things is the right solution for everyone, but it does make sense for a lot of use cases.
Most of the time, I'm on laptops or a desktop. All Linux. I'm comfortable with the command-line and have moved away from GUI tools. So, I don't find this workflow convoluted.
Not much of a mobile user and use very few apps. I especially don't use apps that store data on external servers. I use Evernote for storing some information, but not for any kind of planning. I'm sure good apps with self-hosted options exist, but this system works for me, is efficient, and so I don't feel like trying out anything else.
I'm trying op's system. I put the text files in a top level folder in dropbox. So any machine of mine can access them.
For iphone/ipad, I use Textor to quickly access and edit them. It's faster than the dropbox app, which requires taps to get to the right place and a further tap for edit mode. Textor is about as fast as on my computer.