Ranked in these QuestionsQuestion Ranking
Pro Proper kde intergation
Its the only graphical email client which is properly integrated into kde plasma.
Pro Has system tray icon
Good integration with plasma 5
Pro Supports POP3 and IMAP mailboxes
Con Buggy related kdepim applications
kmail by itself isn't the only application which is buggy, the many of the kdepim applications are buggy as of spring 2020.
Con Poor UX
The user experience of kmail is extremely poor comparison to Thunderbird or evolution on Linux.
Con No google support
You cant pass google 'Authentication' for the majority of the kdepim applications, The only support which works is via the kde 'online accounts' under 'system settings'. This only supports youtube and google hangouts integration.
Con Buggy as of 2020
There are many bugs which are hard to track down in kmail, sometimes without any useful logs for troubleshooting. Sometimes the bugs are related to supportive applications like Akonadi, ect.
Akonadi needs lot of ram