I've only used a TA once for a personal booking. Anticipating there might be an issue, I explained I wanted to UUA and made sure the tickets were booked into T and had them issued quickly. No problem, I was told - UUA will be fine. Well, yes it would have been had I not had AA flights on the PNR but I didn't know that rule at the time and so was pretty miffed that the TA had told me it would all be fine. It was even more annoying as there was decent availability and I looked enviously at a half empty CW cabin on both TATL sectors!