I keep trying these (Canary, Polymail, Spark, Airmail), and they all disappoint in some way, and I always end up going back to trusty old Apple Mail. One big point for me: Except for Airmail, none of the new "minimalist" mail apps do plaintext. They all force you to send as HTML. I would love for "rich" email to work, but it's not nice the way current clients do it. In particular, I think it's incredibly presumptuous…
I feel like you're me. I've the exact same experiences and I keep trying something else but they fall flat and often miss some basic (important) features. Always going back to Apple mail.
The only reason I'm not using it today is that I was using the beta, and at one point they did a lot of frequent bugfix releases, and each new release started to require that you set up all your settings (mail accounts etc.) from scratch. I got tired of it and abandoned the thing.
I'm sure it's better now, I just haven't felt like going through all the steps again yet. Airmail does plaintext email, too.