Here's my take on intervals having done just about every interval under the sun. I have reduced my solo workouts down to four different rides:

1) One or two efforts of 15-30 mins at 92-95% of threshold.

2) 45-75 mins at 86-90% threshold

I do one and two a couple of times a week from Nov- March.

3) A continuous 10-20 minutes alternating 40s on at ~125% of threshold with 20s easy.

4) A Strava KOMs ride (most around are between 1 and 8 minutes) that I hit pretty close to flat-out.

I do three and four Mar-April.

Then while in the midst of full-on racing I'll hit all four at one point or another whenever the mood strikes.

Only the KOM ride is done more or less as hard as I can. Everything else is done in moderation. I'm a big believer of consistent, progressive workload so none of my workouts are viewed in the context of just that single workout. Instead they're part of a weeks-long build and are approached as such (meaning flexibility with power numbers and duration as needed).