It was some dodgy accounting accusations/reveals https://arstechnica.com/gadgets/2025/04/backblaze-responds-to-claims-of-sham-accounting-customer-backups-at-risk/
For me it was an excuse to try something new and cover my ass just in case this didn't pan out. But overall it looks to have blown over
Fastmail is hosted in Australia which has some iffy privacy laws thst may affect fadtmail (although fastmail won't sell your data at least) https://www.e4237161d240bc6333d6834ce-19834.sites.k-hosting.co.uk/showthread.php?s=23fc90acb4f52ac90ee43d800bb66a77&t=74082
I have moved to mailbox.org which has been great too. Just offering an alternative in case you are interested in a European host