I pulled our organization off Fortinet a year ago for three reasons - the high cost, the constant bugs in code that either blocked access that should be allowed or general failures during system updates, and the killer - a poor design on their clustering which caused configuration corruption to pass from the primary to the secondary.
Oh, let's not forget the static ram exhaustion that began the cluster failure to begin with. I can understand the limitation of static ram, but the decision to solder that in place (making replacement impossible) meant that particular unit has roughly a 4 year lifespan.
I am well aware that Fortinet has a strong reputation at the high end, but for midsize shops there are more affordable options that seem to need less hand-holding.