In 2012 I used a third-party site to book international flights on Air Canada and had many problems resulting from missed connections (their fault). Ultimately I got no compensation and at one point one of their reps said to me something like "well what do you expect; you didn't buy the ticket from us directly so we can't do much to help you".
I've just returned from an international trip booked directly from United. When one connection was at risk (their fault), they sent email saying that if we did in fact miss that plane we now had reserved spots on the next flight. We never contacted them (we were busy sprinting through an airport); they initiated that.
Is the difference in these outcomes the direct ticket purchase, or is it the different airlines? (Maybe United's customer service is just way better than Air Canada's?) We also bought travel insurance this time (didn't know to do that the first time), but since we never invoked it I'm not sure that matters.
I'm trying to figure out if the lesson here is "buy direct from the airline" or "fly United (where possible)". I don't have the fortitude (or travel frequency) to determine this experimentally, e.g. by buying a ticket directly from Air Canada to see what happens.