The pain points are what other commenters have said:
- I don't find the default config a good fit for me, and run it heavily customized. As someone said everything in Emacs turns into a project...
- Performance can be an issue, especially indexing new mail (and especially if you like to lug around a copy of most of your emails locally as I do). On a laptop while traveling this used to be more of a problem, but newer versions are notieably quicker and newer laptops have better battery life.
- HTML rendering isn't great. Thankfully I don't get too many important messages that isn't just plain text. This might be a reasonable use case for xwidget-webkit though I'd imagine there are security/privacy issues to work out. (Another Emacs project -- yay!)
When I started I thought it would be an efficient way to get through lots of emails, and it has been for the most part. I'm just not sure I've saved time overall unless one counts the hours configuring it as "entertainment / hobby" rather than "work".
Also, the real test would have been my much more voluminous work email!
The HTML rendering isn't great, as you said, but you are two keystrokes from opening that email in a browser, if you have to.
And I have tweaked the config several times now, but I think that's mostly because I'm changing my (and the charity's) email, which involves a lot of shuffling about. Again, in six months, I'll have another look and decide whether it _really_ helped.
I've not seen the other things you mentioned. I only check for email every 10 minutes, but opening and (especially) searching for emails seem much faster than doing it in Gmail. Plus, I can do searches across email accounts, like all unreads across all three accounts. That was definitely slower in the online clients.
Finally, there's a quick ('a' then 'v') way to just open a message in a browser if the HTML is too thick.
Why not? Does your job mandate that you watch your inbox constantly, and respond immediately to all messages? How do you get anything else done?
This is the reason I haven't tried all the email tools that seem fun to play with, but not worth it :/
And the search.... fantastic. Best email search and virtual folder capability I've used on any platform.