Disclaimer I am not a scientist but I've read stuff on the internet so basically the same.
Estimates without power data are just based on formulas based on age/weight/gender/time and for whatever reason are always high. (Btw Wahoo will work without a HRM it just spits an estimate at you though.)
Power meter numbers are also based on an estimate but it's much more accurate and the difference between power input at the power meter and actual calories burned is such a small number that it's effectively insignificant and the only way to truly measure calories burned is in a lab with a whole bunch of crap hooked up to you.
When I started riding a smart trainer in the winter that had actual power numbers I got a good idea of what I'm burning in an hour -- about 600 calories per hour, but 700 for a really, really hard workout. What I've noticed when I compare that to the estimates when I ride without power data is that they're usually about 2x as high as my actual averages. If I'm counting calories I take that into consideration. (But I don't count calories any more because it's depressing.)