Despite webmail's many flaws, I haven't found an MUA that's as good for me as the Gmail web UI.
- Most MUAs choke on my archive. Simple searches take seconds.
- Every modern email service has a tag-like construct. Representing tags in IMAP duplicates messages.
- Decades later, IMAP sync is still slow, unreliable, and annoying to debug.
- A surprising number of MUAs ignore account boundaries when integrating with my contacts.
- Depending on my job, I go through years-long periods with lots of calendar-related mail. Tight integration between Google Calendar and my mail client is very convenient.
- I use snoozes and (especially) delayed sends often.
- I use all these features on my phone, too.
I haven't found an email client that solves all these problems, on both my laptop and my phone, that's better than Gmail web UI + Gmail iOS app. With two exceptions, all the clients I've tried are slower and less reliable, and they often have fewer features important to me. These days, they're often a less-polished, poorly-tested website in an Electron container. I'm happy to pay for GSuite (or whatever they're calling it these days), because I get ad-free clients that work for me and excellent uptime/deliverability.
* The only genuinely good clients I've found are Mailbox (never shipped a stable desktop client, but had useful innovations) and Superhuman. Mailbox was discontinued post-acquisition, and Gmail has implemented their best features. Superhuman is fine, but not worth paying for (though I'd perhaps feel differently if I spent hours each day in email).
I use webmail + Firefox containers to access multiple inboxes at the same time. I have like half a dozen email accounts that I access regularly, so it works well.