Except Sieve rules in Fastmail either work or do not, and it is non-deterministic. A few notoriously persistent problems I have been facing with them for years:
– A notorious spammer that has been spamming me for over a decade that the Fastmail spam filter does not trap[*]. They are easy to screen out via Sieve rules using the "From" header. Except, for 1 out 5 spam emails from them the Sieve rules do not fire, and the spam gets into the inbox.
– Simple email filing into different folders using From/Subject headers. It mostly works except for just a handful senders whose emails Fastmail Sieve do not fire upon no matter how I modify them. They just. do. not. work. I am talking about the most basic From/Subject filing rules.
[*] Spam filtering in Fastmail has been another pet peeve of mine. It is average at best, and mediocre on average. Some spam emails that slip through need to be sent into the spam folder (that has the training enabled) for more than 10x times before*