T-Mobile employee and Cybersecurity student here:

As long as you have your own good practices, like not giving out your password to your whole family, updating your PIN every year or so, and have some common sense, you are perfectly safe with T-Mobile, Verizon, or AT&T.

They're going to tell you whenever a breach happens, because it's important, but most people don't even know what this means.

No, none of the breaches in the last several years have resulted in personal or financial information being compromised, but things like email addresses, names, and phone numbers could have been - which is the information they're after anyway.

Stealing your financial info is hard - nearly impossible at T-Mobile even if you had someone on the inside. Stealing your marketing info is much easier, and the scammers make more off of selling your email, etc to marketing companies (spam) than they would by stealing your CC number, and with a significantly reduced risk of getting caught.