This reflects only my own experience and view on HR and racing. I have never found training and racing HR to be similar: a HR where I am ‘redlining’ in training can be one that I can hold for an hour or longer in a race.
Skyrace-type events often have such variable terrain that I have to make tactical decisions on the spot that are based on how I feel, such as when the group around me splits into a slightly faster pace and slightly slower pace, and when and where in the race that split might be; whether I am on a steep, hands-on-knees 200 m climb or steady, semi-runnable 1000 m climb, what terrain follows that climb – if it is a downhill then I have some recovery time, but if it is a highly runnable section I do not want to come off the climb maxed out; is it 21 or 42 km (or longer), etc. None of these mimic training conditions and are not captured in training HR, so it would be hard to know what HR to use and at which points in the race to use it as a limiter (my racing HR can vary by ?60 bpm, and more if I include feed stations). The only time this approach failed was my first mountain race 11 years ago: it was awesome for 28 km and awful for the remaining 15 km. HR was not the problem.
In races when I have worn a HR strap, I look at HR only from a curiosity perspective (observations such as ‘wow, didn’t know I could maintain that HR for an hour’), and the race (50 km, ±3000 m vert) where I probably came the absolute closest to a perfect race in terms of where I think the limits of my abilities are, I took an old watch without GPS or HR.
Some discussion of this also occurred previously: