I don't mean this in the way that Client A > Client B, but I have spent a fair amount of thought on this, and I have not been aple to pinpoint exactly what characteristics makes the difference for me, which I find to be interesting in itself.
I noticed that letting mails linger in sight in my inbox is energy draining or causes me to procrastinate.
This is unacceptable to me. Yet I keep using it because I dislike Gmail’s web interface and my Vim imap setup is not really usable yet and probably never will be.
I've used many, many different clients over the last few decades (yes, including Linux `mail`). I have to use Outlook at work, and I have to use my providers' web clients on my Windows gaming PC, but on all my other devices I use Mail.app.
I just... don't ever use all the features of the other clients, or don't like some of the behavior they have, or any of that. For a long time I would get excited about new email clients and try them out right away, but no more.
I dunno if it's just that I'm getting old, or if I just don't care as much, or both, or something else.