Using a GPS watch in events

Perfectly fine.

Not only acceptable, but wise.

Even the pros often will use it to ride to power etc.

I use a cadence sensor, heart rate strap, footpod etc.

Perfectly fine but I would be cautious about being too ridged with HR zones in a race. Nerves/excitement are likely to elevate your HR, making it appear you are working harder than you actually are.

Thank you all. I use an Ambit 3 to train and I think it will just help settle some of those nerves you've mentioned (you know, like: "no, your HR is not 273 bpm" or "yes, you are actually moving"). A bonus is that I can actually find my way back to transition after I get lost on the route - lol.

And just know that all companies program their devices to glitch up during actual races. They run out of batteries, they stop recording because the memory is full, they lose signal, etc. I always find myself riding in aero one handed trying to fix my watch. But then I chill and move on.

I don't have a power meter on my bike so I'll use HR to gauge my biking efforts while racing.  I've correlated HR with RPE on the bike when training but have a tendency to want to go faster during a race than I should so it helps to hold me back a bit.  However I run by RPE while racing. My fastest Sprint runs off the bike have come ignoring the HR/GPS pace and running scared that someone in AG is right behind me!

I also used HR on my first 70.3 to make sure I didn't get too excited during the bike.
Worked well for me, but then again my only ambition that day was to finish within the cut off. So I was ok with potentially not going at top speed.