The ability to cast one's spell on time is pretty much the most important thing
So much so that some players have started to use the rules for the mobile game while playing the physical version
The structural weakness is obvious: everything sourcing judgment, authenticity checking, packing, and your money rides on one person, and rarely survives that person getting busy or moving on
Fine to play as good all round utility filler come removal but not something you can overly consider as a threat which is a very rare thing to be able to say about a planeswalker
Assault Intercessor can take huge chunks out of opposing life totals when combined with the right cards, whether thats dedicated board wipes, a flurry of targeted removal, or just straight-up actual Massacre Wurm
Either way, Mirrym, Sentinel Wurm is an amazing Dragon card, and capable of taking down Commander pods at the helm