Are there any IFR training plugins / addons for Microsoft Flight Simulator or X-Plane?
Both sims allow you to do a review of your flight where you can see both plan views and vertical profiles of the flight to analyze the results.  This can be especially useful when analyzing the results of attitude instrument flying like A and B patterns, etc.

CloudAhoy does allow integration of flights from X-Plane for review as well.

ATC is a bit better in games like FSX and Prepar3d, but I’d guess there are live ATC available in realistic multiplayer sessions of X-Plane as well.

One real pain in the butt about those Sims is just how out of date they are in terms of navigation databases, etc.  you really can effectively simulate approaches if you can load and practice the latest ones.

That being said the games are excellent to practice basic attitude instrument flying in as computers can simulate that very well sans the vestibular distractions which accompany flying in IMC.  You can also practice partial panel and other IFR emergencies here as well.