I hit my highest FTP last year with a tempo build (mostly 3x20minutes and then finally working up to 4x20 minutes). It’s very fatiguing and you really have to pay attention to the training load and inserts rest periods.
I made the mistake of going 8 weeks without a rest week and then hit the wall. I didn’t rest other than a day here and there because I felt great.
So, IMO, long tempo can be a great builder of TTE and muscular endurance but you need to treat those sessions as they are an interval workout and factor in recovery.
I also have been doing short tempo intervals to break up the monotony of pure Z2. Inserting a series 1 or 2 minute tempo intervals can spice up Z2 so that it is less boring. My mind likes counting while on the trainer. Like 1 minute tempo, 4 minutes Z2, repeat every 5 minutes for some period of time.