My experience with Fortinet products has not been great overall. A local VAR who used to sell Nortel started moving their phone systems as a segue into the IP phone market and every single one of these units was a disaster requiring regular reboots even with just plain analog lines connected. With IP (SIP...etc), performance and reliability were even worse.
They then started moving the "next generation" ones and these weren't much better/almost as bad. Support was atrocious and none of these issues were ever resolved. They ended up pulling most of them and replacing with Avaya.
Not long after this the local Mercedes dealer received a pair of Fortinet firewalls to replace a single Juniper SSG. Already being a bit apprehensive of their product I questioned the MSP about this decision and they said "yeah, that's why we sent two, they are still cheaper than a single SSG".
I come from a mostly IOS and ASA background and was looking for something easier to manage remotely with better visibility and Cisco-level reliability. Meraki has been a good fit in that role. While the MX family lacks the complexity and deep configurability of something like FirePower (or even Sophos XG), if it works in your application (and I've found a lot of the time it does), it may be a better fit for some of the reasons @DHAnderson touched-on. Meraki equipment is wickedly simple to keep up-to-date and manage and there's a security benefit to that which needs to be considered. Also, in my experience, Cisco's support is far better.