I've used Thunderbird for many years, but recently switched to KMail because that has proper search functionality. In KMail, I can instantly search for a keyword in all my emails from the last one and a half decade, whereas this takes several minutes in Thunderbird, and during the search the UI is unusable because the result list is incrementally populated which makes it impossible to actually click on results reliably, or even just scroll.
Other than that, Thunderbird works great. I'm just puzzled why search has not received more attention, as it seems like a pretty important feature to many people.
edit: I know about NotMuch by the way, and actually used that to search and then navigated to the result in Thunderbird afterwards by sorting by date. But that got annoying in the long run.