I do intervals using HR. It's more difficult than using power, but one can learn how. One has to learn to do intervals with power, also.
Say you want to do 3X3', probably the most difficult to get right using HR. You warm up really well while riding to your candidate repeat hill or straight stretch of road. You start out at what you think is probably the correct effort, watching your HR. If it stops going up short of your target, increase the effort. You're going to be panting, so use that as a guide, too. "Controlled panting" I call it. At the end of 3', there's a good chance that you'll have achieved your target HR for these intervals. Note your gear and cadence, gear down and do your recovery minutes, then return to your end-of-interval gear and cadence and watch your HR again. It should come up much faster this time. If you should go over your target this interval, cool it down a hair, but you probably won't. If it doesn't quite make your target again, keep increasing the effort until you're there. Repeat for the next interval, etc.
Not as precise as with power, but that's a quibble. Works fine for getting the appropriate training effect. Caveat: if you can't hold the HR for the full length of whatever interval, you're not ready for it yet. Get more base, more lower effort intervals.
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