Just two things that spontaneously came to my mind:
1) I guess Meraki is simply too expensive for an ISP to hand out to their (end) customers. Especially regarding the fact that the lower end boxes are not able to cope with current circuits (in many countries)
2) Depending on using Meraki MX in the managed SD-WAN space, there are at least two things that are real showstoppers for many environments: 1. Lack of IPv6 support, 2. No possibility for VRFs
On the other hand, there are quite a few MSPs in here, so something e.g. centralized and simplified management has to make Meraki a good choice for them. Guess one or two will chime in here 🙂
A number of MSP/ISPs over the world offer Meraki as a solution, 99.9% of the time as a managed service. This can either be driven by a customer requirement for something like SD-WAN, or from a management simplicity aspect.
In terms of benefits, from speaking to my partners who role out this as a managed service for customers, it's the ease of deployment, visibility and the ability to be more agile and proactive. One example of the latter is using some of the intelligence offered by the platform such as Meraki Insight. This allows the individuals managing the estate to get really granular stats around how the underlay network is performing and if there are problems accessing specific applications/resources along with many other things.