I'm not sure, but I believe you don't lose visibility of the Syslog.
But you do lose visibility of the global Layer 3 firewall rules, because those rules stop being applied when a Group Policy with custom firewall rules is applied.
You still see client flows, URLs, security events, etc. in the Syslog, but they reflect the Group Policy rules, not the global Layer 3 MX rules.
At least that's how I understand it.
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
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