MPTCs are not controversial.  They're also of limited (but extant) usefulness without an additional extension; and, while there is "controversy" there, it's not especially relevant until type families are stabilized.  They could in theory go into the standard *now*; they'd just be of limited use until functional dependencies vs. type families is settled.  (Also, de facto I think it's already more or less been decided in favor of type families, just because functional dependencies are (a) a bit alien [being a glob of Prolog-style logic language imported into the middle of System Fc] and (b) [as I understand it] difficult to verify that the code in the compiler is handling all the potential corner cases right [mainly because of (a)].

In any case, if the code in question doesn't happen to need either functional dependencies or type classes, the controversy doesn't touch it.