I can't see how Sieve is significantly better than Gmail rules. Reading the examples, I seem to have the ability to do everything listed, with the exception of actually rejecting messages (versus quietly deleting/archiving).
For one, it's much easier to maintain complicated rules in Sieve than with Gmail rules. This is just a function of having multiple lines and comments for rules.
I use Sieve to push a summary of certain messages to my phone (with https://pushover.net, another great service). This is not possible with Gmail rules (you can forward an entire message, but not selectively send just the subject and sender). See https://www.fastmail.fm/docs/sieve/fm-sieve-notify.html
Note that you don't have to do all that crazy spam stuff -- the build in spam filtering seems to work fine for me. The only manual rule I added was to spam anything with common Russian characters because Russian spam was somehow slipping through the filter.
That’s mostly because the email has already been accepted by the incoming SMTP server by the time it gets to Sieve, hence cannot be ‘rejected’ at the SMTP level anymore.
