Depending on where the MX is to be placed, you‘re stuck either with mandatory NAT or having to run a possibly not „ready“ No-NAT implementation.
From a technology point of view, MX is kinda comparable even to Cisco Firepower: it also runs Snort and uses the same AMP engine (you already mentioned Threat Grid). If you want to have more possibilities than „activate IDS / IPS with a specific ruleset and whitelist rules“, you‘ll not be very happy with it (which is similar to e.g. Check Point from what I‘ve heard). The simplicity on the other hand is unbeatable here.
AMP and Threat Grid are great security products but only if they‘re able to see the traffic. With the current percentage of encrypted traffic the MX will have a hard time „seeing“ / analyzing threats unless the TLS decryption is officially available (if it makes sense or not though). Decryption performance on current MX seems way lower than Firepower though.
After all, there‘s one thing you won‘t ever beat with Meraki: the dashboard as single pane of glass for everything regarding your network which is of course very favorable when it comes down to OPEX.