Lets consider MPLS a little bit further. For the "customer" view it virtually looks like a full mesh.
Physically however it is quite different. Your have the CE router on the customer premise, which then connects to a providers PE router, and then to the providers code. When site A talks to site B it goes CE->PE->Core->PE->CE. The important bit to note is that it traverses the providers core. It isn't actually physically a full mesh. The traffic does not magically physically jump from one branch CE to another branch CE (there is no actual physical mesh).
The provider core is essentially the same as your Meraki Hub.
Just like the service providers core network - you need to make sure your Hub environment has plenty of bandwidth and redundancy and processing capacity to ensure that their are no issues.
I have run a 300 site Meraki network off a pair of MX's as a hub and it runs great. If your deployment is spread over over a large geographic area (such as multiple countries) you could also consider using multiple hubs - a pair for each area. That way traffic within from site to another in the same geographic area will not leave that geographic area.
