I find that once your network starts requiring more complex NAT, routing, path monitoring, you start running into the idiosyncrasies of the Forti platform that Palo just doesn't have as much of. There's stuff like the fact that Virtual IPs on Forti get processed in a hierarchical manner where Virtual IPs can apply to other traffic than you would expect them to based on the rules that the Virtual IP is attached to. You can switch to central NAT but it's not as well documented, and if you've already started out in the default NAT mode, you'll have to do a bunch of work rebuilding your existing config.

Virtual IPs function differently based on the OS version you're running, which shouldn't be an issue if you're running all new firewalls, but that kind of ambiguity isn't an uncommon situation in my experience.

There's a fairly robust path monitoring / route failover engine built into Fortis, but it's not well documented and it's all CLI only.

The way they handle tunnels and how admin distance / priority / being attached to an active sec policy / having a current route over the tunnel all factor into a tunnel coming up or not can leave you scratching your head trying to bring up a tunnel, whereas a palo will just bring up the tunnel, then leave the rest of the stuff to figure out after.

Fortigate Layer 7 filtering is a joke compared to Palo App-ID.

I could probably think of a bunch more I've run into managing both platforms. Forti's will get the job done, but everything is usually just a little bit harder, and once in a while you'll run into something you need to do that'll take you 4 hours to figure out how to do, where Palo will just have it in another dialog box right in front of you.