My problem with working with Google, Microsoft and Apple (and even Amazon) products is that everybody tries to do everything. They all want their own cloud storage, contact system, email addresses, phones, music and pretty much everything else. This is extremely confusing for the everyday user who's trying to find the best product for what purpose (I use GMail, Microsoft Office, and iCloud instead of icloud mail, google docs, and OneDrive for example). I wish in our current tech world that there were industry standards that everybody uses for Office apps, cloud storage, music, news and everything else, instead of having to navigate the mass of services which makes everything so confusing. There's too many choices.
Thunderbird solves this problem for me when it comes to email. Thunderbird is extremely reliable. Its interface is simple and feels and operates much like Firefox so Firefox users will be very comfortable navigating it. The fact that it's made by Mozilla, a third party company means that it's intention is to sync with google, yahoo, and many other types of email accounts without pressuring you to sign up for their own. The calendar syncs nicely with my google calendar, and I can easily use my five email addresses (four gmail, one yahoo) with their client.
Thunderbird's infrastructure is easy to use. The subject headers are positioned so you can see the entirety of the subject so you know what you're opening instead of little snippets like Apple Mail. All the folders sync nicely and Thunderbird even has an add - on for contacts. Mozilla still has that innovative start - up feel to their company and are always so willing to receive feedback, so any potential bugs are likely to be addressed by developers.
The calendar has some sync problems with Google Calenda and Contactsr. The names of the calendars and contact groups haven't properly synced across all my devices (iPhone 6+, Macbook air, and a Windows 10 Desktop I built).
Having a reliable email client across both Windows and MacOS is very efficient. I like coming to one source on both machines for Mail, Contacts and Calendars. I hope Mozilla continues to iron out the bugs and improve on the layout while keeping the functionality steady.