Apple has the best unified modem support, so aside from select global regions (very few) you can absolutely travel knowing your phone will get network coverage.
Samsung is still kind of fragmented depending on continent but they are much better then say 3 years ago. If your primary area is the states, then an unlocked model released after 2018 should have all the bands for that Model Year it was released in. If you are tech friendly, sometimes carrier
locked models, can be carrier unlocked, then firmware unlocked to basically the unlocked, carrier free model U1.