Adding to this - ideally this integration would support both QuickBooks Desktop as well as QuickBooks Online. 

We just conducted a 600+ member-wide survey of our club and this was cited as one of the biggest issues across all categories for our members. We send emails/texts out notifying members of upcoming events with the event registration link, and on mobile devices the link opens the member portal website, rather than bringing them to the mobile app which they find both confusing and annoying. A URL scheme to access the mobile app with a deep link is really needed. Perhaps this is something that the new custom/branded app supports? We are using the standard mobile app.

Same here - this is one of our biggest sticking points right now. The built-in mail platform is too basic to use for our club, where we want to be able to track clicks/open rates, scheduling of emails, templates, etc.