I think most of the complaints are about the user interface nowadays. It sometimes does have other problems, but that is probably because it takes a while for Yahoo to change.
After 12+ years of service they deleted my secondary email account because I hadn't visited it in less than a year ("username has been recycled"), then wouldn't let me re-claim the username, and have no support contact for it. I will never trust them with anything important.
You made me launch my old Yahoo account to be sure it's still there.
Then I tried to respond to an email in Yahoo! Mail, and EVERYTIME I HIT THE LETTER 'i' IT POPS UP A CHAT DIALOG BOX WTF. Sorry, but it's really frustrating.
Both the user interface and the spam filtering are awful. I only use it for a throwaway address (I personally don't pay for it, rohandhruva) so I've never tried any more "advanced" features. Why bother if it can't get the fundamentals right?
If you're also referring to the Gmail interface, it's because all the desktop clients I tried suck - this year alone I tried Thunderbird, Apple Mail, Microsoft Exchange and Mailbox and I can't even remember the apps that I tried on my iPad or on my Android. They all suck compared to Gmail's web interface.
Out of curiosity, have you tried Airmail on the Mac? I tend to agree with your observations about the clients you mention, but Airmail really stands out as excellent to me (although I like version 1 better than the recent version 2).