I've used Thunderbird for years and know half a dozen other people using it, including one who has folders with tens of thousands of emails, and have never heard of any data corruption.
I would love to find an actually performant email client. It shouldn't take like seconds to sort like 100 000 emails. It's a puny number. The time it takes one can read all the emails from disk a thousand times, it's sad.
I have never seen any corruption, but if I have threading on, emails get attached to the wrong thread quite often. It's quite annoying.