70.3 Andorra bike course almost has 0 flat section (only the tunnels). Same for 70.3 Nice.
I raced these races on my road bike as did a large portion of the AG field. Pros were on TT bikes (but then Gustav won Nice 70.3 WC on a road bike in 2019).
There are many smaller local races in the French Alps where I used my road bike (Triathlon de la Madeleine, VercorsMan, etc.) as it's mainly up and down

I also raced Embrunman on my road bike but a powerful rider would probably be better off on a TT bike.
I raced IM Mallorca on a TT bike.

As someone else posted, it's really a personal choice

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It doesn't get easier, you just get slower
https://mymsracesironman.home.blog/

The L (main race) is often still won on a TT bike. With 118km & 3.2km gain, it's does seem a close call though.
but the M would be a road bike for sure. 28km & 1.4km gain.

Basically 14km flat into 14km at 10%

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For me, the only reason to use a road bike is if there were steep enough climbs that my tri bike's gearing was insufficient (I can't get the same low gear as my road bike without a complete drivetrain replacement). Handlingwise, I've never had an issue with technical descents on my tri bike, in several cases my PRs were set on the tri bike...

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"I'm thinking of a number between 1 and 10, and I don't know why!"

And got significantly outbiked

 


Loads of people on hybrid bikes at my local tri too...that doesn't mean it's the right choice ;)

Well, the fast people use TT bikes. You have every right to be slower.

For me, I used to have this issue, but when I went to 12spd my gearing became very close and now I can ride the climbs almost in the same low gearing I have on my road bike, so I can think of no situation to use my road bike in a race. I have timed myself down a technical 5km descent and my tri bike time is faster than my road, as any part that requires power, I can put it out whilst staying very aero on the TT bike rather than sitting on the tops or even the hoods of my road bike. I would only use a road bike for race up Le Mont Ventoux or a similar steep terminal climb.

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Triathlon X in the UK possibly. Country roads with poor road surfaces. Gradients of up to 33% up and down. Over 5000m of climbing on the ironman distance bike route. Certainly the old half route that took in the biggest climbs from both sides on a semi out and back type route with not a lot of flat inbetween.

I believe it depends less on the course and more on the competitor. My understanding is that the progression of speed is as follows: aero bars, drops, hoods, and pursuit bars. So, for every moment a competitor is out of aero, he/she is slower than a road bike. Regardless of the course, if you can stay in aero, you will be faster on a tri bike. But anecdotally, I think we can all recall seeing age groupers hanging out on their pursuit bars for prolonged periods of time. I think those folks would be better off on a road bike, regardless of the course.