We've been using the 15.x train for over six months for our UK WAN and customer internet edge. We haven't had any unexpected issues across 9 sites, thousands of users and ~25 MXs.
Big caveats:
We don't use the advanced license features
We don't use cellular or Wi-Fi models
We don't use BGP/OSPF in production
We have MX64,65,67,84,100&250 models.
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