I'm in a city where there are a handful of one-day comps, and I have traveled to attend a one-day comp or two.
In my experience, there are the really super nice one-day comps (the Type As), the ones where the organizers are working hard to provide a beautiful and well-run experience, participants (and judges) come from all over, the hotel ballroom is lit beautifully and is decorated gorgeously, and the day ends with dinner, some social dancing, and a pro show that evening. From the look of it, you'd hardly know it was a one day.
Then I've danced in one days that are in local venues (Type B), where the judges are local, it's mostly just local dancers, there's no "decorative atmosphere," the crowd and the participants are warm and supportive, and there's social dancing that evening.
Then there's Type C, which is a Type B working hard to be a Type A, and all that will take is time and more word of mouth! I find these the most fun of all!
But all types of comps have their places - if we're trying out new choreography before a big multi-day comp, we'll do a one-day and it doesn't matter which type. If there are six months between big comps, we might find a one day three months into that break so that we stay "competition ready."
What I noticed over my tenure dancing pro-am is that a non-local, Type A one-day can be as expensive as a non-local, multi-day comp. Package fees for three such one days (I've danced at all three) have package fees that are equivalent to at least one multi-day comp I've attended. So when this happens, you look at a calendar and your bank balance and you make some choices.
