HOPs or microintervals in all their various forms are great if you want to hit both sustained metabolic power AND the ability to repeatedly accelerate (neuromuscular) and a bit of anaerobic tolerance in the same session. Really good stuff to prep for dynamic riding that puts an emphasis on frequent accelerations to cover gaps, come out of corners, respond to field surges, etc. Also pretty good for breaking up the monotony of iso-power indoor training as something is changing every few minutes so there's not much time to get bored.
But there are many, if not infinite forms of sustained microinterval work. Bill's classic HOP work as you've described it is awesome but it is on the harder side of things as you've pointed out and just mentally it's probably best not to try those too often or you risk getting into workout dread and skipping sessions rather than doing good solid work. IOW, it's a bit like chasing records in every 2x20 session, fun for a while but can easily lead to training burnout and it isn't really necessary to set new records every time you train when good solid 90-95% of FTP still provides plenty of training stimulus.
So in terms of microinterval work, consider variations on the theme and chase the real hard stuff from time to time but do solid but not record setting versions on a more regular basis. You can basically play with three things: base pacing, burst frequency, and burst duration and to a lesser extent burst intensity but for short bursts there's not much time to consciously pace and it ends up being whatever you can rally to do on a repeated basis. So some workouts I use a lot are things like:
- 'Tempo with a twist': where the rider sets a base pace in the mid to high Tempo range and then bursts quickly every two minutes for a very short leg speed focused in the saddle sprint. Starting out we'll usually target 6 second bursts with huge emphasis on quickness and leg speed not grinding up to higher power and often not shifting but really whipping the legs up to speed quickly. Then the goal is to get back to base pace as quickly as possible after each burst. These emphasize on demand muscle recruitment, rapid accelerations, and recovery in motion between bursts. Over time we tend to stretch the bursts to 10 or 15 seconds but always emphasizing the acceleration and quickness aspect. These usually end up with an NP in the SST to low L4 range depending on how you do them and are typically sustained anywhere from 15 minutes to an hour or more. They're not too tough mentally and can be done fairly often.
- Classic longer burst HOP work as Bill describes but again you can vary the base pace to make them work as well as the duration of the bursts
- Over/Under style work with blocks above and below target power (typically above and below FTP) but that average to the target power for the effort. So if targeting 250 watts average it might be two minutes at 270 followed by two minutes at 230 sustained for the overall duration or something along those lines.
- Lot's of games to play like doing the first ten bursts at say 10 seconds, then the next ten stretched to 15 seconds, then bump them up and so on until the final bursts in a microinterval set are long and brutal or do them pyramid style where you ramp up the burst duration and then ramp the bursts back down in length but keep the frequency the same, keep the base pace the same and try to keep the quality of each burst very high whether long or short.
Bottom line, tons of ways to craft interesting and useful dynamic indoor workouts. If you want to actually complete them then the AP and NP for the sessions needs to be within the rider's capabilities and how difficult the workout will be from a recovery or even mental perspective is pretty closely related to session AP/NP. So in the case of classic 30 second bursts with a base pace of 91% FTP you're looking at a very challenging workout with NP near FTP for a full hour. Don't do that variety too often or at least any more often than you might try to do a full hour right at your FTP, that's very hard work and a good path to burnout.
Good luck,
-Dave