The only time I've encountered an issue was when trying to check a car seat, base, snap-n-go (stroller frame for the car seat) and a regular stroller. We were counter-checking the stroller and base, but going to gate check the snap-n-go and use the car seat aboard since we had a seat. The counter agent gave us a hard time about only being able to check a car seat + base OR a stroller (including a 2 piece) for free.

The resolution was simple - I let her lecture me for a bit, as she was condescending in tone, then asked her to finalize her decision because we were flying F and only had 2 bags for the 3 of us to check anyway and that we'd call it luggage if she preferred. That's the only rude US agent I've encountered this year, and it was in SAN of all places.