I never did IM and just did one HIM.
In general, racing is always very though, while finishing is always just a matter of time.
Let's make some examples.
Think about a short, hard race, like...5km. If I ask you, would you prepare for 6 months a 5k race under 17minutes or a marathon just for finishing, what would you answer?
Training for 5k race under 17 can be hard. But everyone can close a marathon, again, just a a matter of time.
What about if I ask you a 10k race under 36mins?
So another step before your question.
Let's take finishing HIM versus racing a standalone half mary. For me, racing a standalone half mary with the goal of beating my PB is very though and I need to run at almost the LT threshold to do that! Closing HIM is easy, you can relax during the cycling and then running as an easy jogging.
Translating to HIH race vs. IM finish, I would say the same even if I am preparing for my first IM. In general, if I prepare for an event, it is never just for finish. Maybe at the end I will walk but that is another story, story of failure.
Maybe I go for something like 12hrs but if I need just to finish it, I can get 16hrs which means I can take the cycling part much more relaxed and the marathon will be run + walk. If you go for such an event and you are not sure to finish for physical condition reason, better not to go. Thinking of not closing it because of EXTERNAL condition, it is normal and you can not do anything with that. It is IM overall, and in 15hrs everything can happen!