There are a lot more than two apps which read or write FIT files for purposes other than workout planning. I wrote one of them.
I specifically called out workouts.
one tedious thing is needing to manually create the workouts. Unless you're doing the same thing week in and week out, it gets old quickly.
I'd build something like a ZWO to FIT converter but there's no way to load them onto a bike computer without Garmin or TrainingPeaks
That all being said, I could imagine more complicated workouts that surpass simple lap/leg tracking could be problematic.
I personally highly recommend intervals.icu