Features are platform and carrier dependent so you should be aware of what isn't supported.

You might also want to search to check for wifi calling as that's useful for calling overseas numbers while overseas on wifi (free to many countries even on the inexpensive pool plans).

Apple - everything should work on Verizon/Warp
Android - everything should work on AT&T/Darkstar. Some Android devices work better with a psim than esim. Manufacturer factory unlocked (vs carrier eventually unlocked) or by manufacturer can impact this so possibly chat CS before signing up and activating.

Earlier thread which also has roaming info US Mobile Users, International Roaming
International roaming features are currently carrier specific https://www.reddit.com/r/USMobile/co...ional_roaming/

As @kirkbauer mentioned, switching between them is not seamless simultaneous access. Those who are thinking of Google Fi's dual network one (secretly two) phone number, this isn't it.

USM also changes their plans seemingly every 4-5 months by adding plan features (minutes, roaming countries, pooling, etc), removing plan features, and discontinuing entire plans. If you see one you like, subscribe monthly and stay on it. They retain the right to change existing subscribers features for Annual plan. You'll probably be happier if you don't chase too many sign-up promos, referrals, or gimmicks as they seem to have gotchas. ETA: or promises of "coming soon" as they push releases out beyond ETA with regularity.

Customer support is ok for a MVNO, imo. There's complaints of when things go wrong, first line chat reps might need to escalate or you have to take to reddit USM staffers. I feel like I get what I pay for and a bit more as the app/website show data usage details (some MVNO do not) and support can be contacted 24x7 via chat, e-mail, and phone. I use the inexpensive "Shareable Data" aka Pool plans and buy travel esims for international data.