I think there's a tension between explicit and implicit organisation. If you want a really good email client, Fastmail is great, but while I did care about that at one point in my life, what I want now is really just the big box at the top of the email telling me about my package arriving today or showing my boarding pass. Fastmail is excellent for explicit organisation, but its implicit organisation isn't great. Gmail is the other way around. I don't like the Gmail UI or any of the organisation features, but I also don't really need to use them.