Where I have power, I use the kj/ Kcalories.
Without power, I’ve had the expected weight loss/ kept at maintenance using Garmin with HRM, for rides without power and other activities (running and walking mainly). That was with the 310xt, 920xt and now with the 245 wrist based HRM.
My Wahoo Bolt can be a 1000 calories higher than my Garmin watch for a 4 hour ride, both based purely on Heart Rate, with the same data/ zones (actually, if anything the Wahoo is a lower weight, as my Garmin is always that days weight). For my non-power rides I run my Garmin 245 for My Fitness Pal, and is one of the reasons I’ll upgrade to the 530, not the Roam, when the time comes. The Bolt doesn’t even give calories without Heart Rate, even with power!
The thing with anything Heart Rate based, as well as the other possible fluctuations, you have to have semi accurate zone setting. Max heart rate in particular, not just some formula.