This is only a supplemental test to how CF’s traffic management works. Don’t take the follow list as concrete facts, this is just what I’ve discerned from observing:
For free plan websites that are “heavy”, eg. take up a fairly good amount of bandwidth/load but not to a point where they’re kicked off the site, I wouldn’t be surprised if they get routed to the “not overloaded” data centers to take the load off of the really high-priority datacenters that Enterprise, and usually Business customers depend on.
On the business plan and enterprise plan, CF will generally always give you IPs that route to the closest datacenter.
Only in extreme cases (such as a huge regional DDOS attack) will even Business plan websites be routed away from the closest data center.
The only time this has happened and I’ve experienced it was about a month ago when requests going through ATL (Atlanta) had an extra TTFB between 2,000-10,000 milliseconds. Checking that page periodically, I saw free websites routed to Alabama, then pro websites, and about 10 minutes later even Business websites were going to Alabama and North Carolina. Enterprise websites stayed on ATL and the TTFB went back to normal (less than 5ms added to each request), until about an hour when everything was routed back to ATL.
The test page I set up is only meant to be a way to observe how the routing is working at the moment. It doesn’t represent how CF will always work since traffic is variable and CF may need to re-route to prevent data centers from going down for their highest paying customers.
For your specific case,
Is your geographic generally Australia, Africa, or South America? These have fewer data centers than NA/Europe so I would understand where they’re coming from when they say this if you want 100% all the time same continent routing.
If this isn’t the case, say your visitors are mainly NA/Europe/India, then this makes me think that they couldn’t reasonably suggest upgrading to the Business plan since you have 100+ domains. The only plan that can be purchased for multiple domains is Enterprise, so suggesting you upgrade to a Business plan for all of them would be cost-inefficient and would get you fewer features overall. I’m not sure about how much exactly Enterprise multi-site is, but it might not be as expensive as you think. As you said you were talking to Engineers, I would suggest actually contacting Sales and they’ll be able to better estimate how much your Enterprise plan would be.