The Enterprise plan offers many additional integrations with third-party applications, such as Microsoft’s desktop Office products as well as the cloud-based integrations available in the lower-tiered plans like Pipedrive CRM, Zoho CRM, Asana, RingCentral, and DocuSign. Therefore, it’s best for teams that want the security and flexibility of Gmail but will be using desktop office tools.
One of the most important compatibility factors to consider with email is the mobility question. How often do your employees need to access email via mobile devices? That's an important issue because most email hosting providers deliver some kind of web client usable as a default inbox. Almost all of these can be accessed via a mobile device, so if your employees don't need to access their emails on the road that much, then such mediocre clients are probably fine.