Thunderbird doesn't currently support Gmail Inbox Categories. This filtration is offered on all Gmail accounts and works in the background without any user input to effectively keep the Primary Inbox free of 'automated' messages.
The significance of this function can't be overestimated and the lack of support for this is the main reason I can't use Thunderbird. Sadly, Google doesn't expose this categorisation via MAPI which is probably why this feature isn't available in most third-party email clients, but there must be a way to implement it because it's available in Canary Mail (see screenshot).
Canary is a proper desktop app, but has none of the power and flexibility of Thunderbird so doesn't offer a workable alternative to Google's browser-based Gmail client, the limitations of which will be immediately obvious to any user coming to Gmail after using Outlook/Exchange. A desktop app that offers a similar level of functionality and usability is desperately needed by business Gmail users such as myself.
Thunderbird is currently the only option available that offers a UX comparable to Outlook and is arguably a much better, but the lack of support for Inbox Categories makes it a non-starter for me.
If Canary can do it why not Thunderbird?