Just wanted to share one option I recently found out, not many people aware about.
To start with, I am working in a large insurance organisation (which is in much regulated industry) with a pretty complex configuration of load balances, reverse proxies, multiple geographically distributed CD boxes in different data centers (and networks). So I was very surprised to find out there is such an offer from the vendor.
Sitecore offers to host you xDB in their cloud, powered by Azure. All maintenance and processing raw data from Mongo to Reporting database is done on their side. What is ends up for organisation is just setting connection string (to reporting database) in config and ensure firewall rules allow connectivity to the instance.
Pricing seems to be bespoke for your solution and not cheap (as everything from Sitecore), but it is reasonable if compare to a full time resource efforts, for example it comes out that our company even saves a bit! Again, this may not fit to all organisations and depends on people and infrastructure they already have in possession.
This information is a very "early bird" for me, so I will update with more details as soon as we start working with Sitecore xDB Cloud