Many MVNOs have "unlimited" plans that are definitely not unlimited. If a plan offers 50GB of high-speed data and then throttles the connection to 256kbps, it's a 50GB plan. By contrast, a plan like T-Mobile's Go 5G Plus/Next plans never slow your mobile data.

Other plans like T-Mobile's Go 5G (regular) will be lower priority once you've used 100GB. That lower priority averages 11-15% slower. Most of the time, the network isn't congested and it doesn't matter. You get virtually the same speed as truly unlimited plans. 11-15% slower isn't the same as getting throttled to 256kbps. Of course, it will depend on network conditions. If you're at a fireworks show or a music festival, lower priority might be a lot slower.

There are options if you want truly unlimited mobile data. If you're looking for hotspot, that's harder. T-Mobile does offer its Away Unlimited plan for $160/mo, but that is lower priority - 13-30% slower at the 25th and 75th percentiles. 77-292 Mbps from the 25th to 75th percentiles is pretty decent performance, but if you end up in an area with lots of network congestion, then you're low priority.