I wouldn’t say there’s a catch, but there are quirks. I was with Virgin Mobile for a few years. Great value, but I got tired of the mediocre phones, and you can’t bring your own.
I switched to Google Fi a few months ago. Google Fi you have to agree to let them use public wireless networks (they do encrypt your data). One upside is that you only pay for the data you use, so if you prepay for 2GB of data and only use 1.2 they will refund the 0.8GB on your next bill. And I love my Nexus 5X, although another quirk is that you only have the 2 Nexus models to choose from.
I recently got a Republic Wireless phone for my 7th grader. $10/month for unlimited talk and text but they also rely on wireless networks to cut costs.
We may move to Cricket once my daughter gets a cell phone as they have discounts for multiple phones.
I hate looking around for a new no-contract cell phone plan because you really have to research each one and compare. But I find the no-contract providers are a much better value than the contract providers.