For sure you can still do intervals with a HRM and a little experience. Once you are warmed up properly, unless your intervals are very short, you will be within the right HR zone for them pretty quickly I find and it will feel right, you will know roughly if you will be able to hold that effort for the intended duration. What is much harder is say going very long intervals. Say you want to do 2 x 20 minutes probably without a power meter you would go too hard the first 20 and not hit the intended effort on the second. At least for me I think this, because doing this with a powermeter I always feel a little slow, but towards the end I am thinking, ok had I gone harder at the start then I would really be suffering now.