There's no accuracy advantage to running Munchkin via GTM, only disadvantages. There are (to some people) manageability advantages, but I care about accuracy first.
What people fail to reason through when adding analytics scripts (I detest the term "tags") is that if analytics is mission-critical -- and every CMO says it is -- pageviews, especially the first pageview on a device, must be logged before the lead navigates away from the page. Yet the more preconditions and dependencies you add before Munchkin can finally load its tracking pixel, the less seriously you're taking analytics.
So these are clearly conflicting priorities. If you want processes to complete before the person leaves the page, you want them closer to the synchronous, must-complete foreground. If you don't want scripts to ever interfere with page rendering, nor vice versa, then you want them in the asychronous, best-effort background. You can't have it both ways.
There's rarely a practical problem on high-speed, low-latency connections, though. The devil is in low-speed, high-latency connections. The 3G connection, the train in a tunnel, the spotty shared wifi, long geographic distances. Built-in network limitations of mobile devices make it even worse. If you serve 30 asynchronous scripts to a smart phone on a low-quality connection, don't expect all the tags to their jobs.
GA has some techniques that make background analytics more reliable (but then GA's servers are more reliable in general). My Munchkin Enhanced library adds some of those extensions to Munchkin, but it can't cover all the bases. I always recommend that my clients use the simple embed code (which removes only one async layer -- there are still two required async calls), or, even better, load the tracking pixel directly (this requires server-side coordination).
In all, we may be talking about the difference between 99.99% and 99.999% of requests not completing. However, if you're generating 100K requests per day, that's 900 misses, and the failures are going to be weighted toward first-time visitors with unprimed caches -- people you definitely want to see.
Look, it's not the end of the world, but I think once you're aware of the risks, you want to mitigate them.
.png)