In my experience with OTAs they have always provided the correct result at the end of the story. But a couple times needed several calls and long wait times. And by "long" I do not mean 30 minutes, I mean hours.

But these were with Chinese airlines that Expedia/Orbitz had to "long book". I.E., they had to actually call the airline to make the change.

With DL (and I assume most) they can handle this via their own IT. So I would not expect a huge issue. But if the online request fails, expect more of a wait than you would with DL.

I think there is no risk other than annoyance.

I also think DL will likely be able to book it if you call. That would be my choice.