I used an OTA once and only once in recent memory. It was to book a fare that was valid per the fare rules, but would error out at checkout because of the very long layover. (It was not end-on-end, it was a single fare that somehow didn't disallow a long layover in between) It was fine, and I got on the flights without a hitch, but honestly, I would only do it again in extreme situations like this, and I would have a plan b just in case.