IMO their iOS app would be great if it had offline support - it doesn't...
Because on Mail.app (iOS/Mac) if you want to reply from that random-email-addr-smjgg@ you have to first add that in "Email Address" for that account in Settings. I am trying to find an app that does it (my mail provider is not fastmail, it's mailbox).
Lack of this feature makes full use of catch-all very unintuitive.
For new messages you still need an extension like Identity Chooser or to manually edit the from header before sending.
Otherwise I've been super happy with them.
A different client is definitely the fallback, though I haven't found a great one of those either. Granted it's been ~6 years since I last searched.
Open to suggestions!