Earlier this year we were traveling PDX-SFO-LIH with our 1 year old as a lap child.

There were empty seats on SFO-LIH, and I got the UG, but there wasn't a single empty seats on PDX-SFO so they wouldn't let us gate check the carseat to SFO and thus we had a seat we could have used it in for the long flight and no car seat. getting that thing through security etc is such a pain, I'm now always checking it (its free BTW, no baggage fees if you would otherwise have to pay them).