[asdf1234]

Pervasive use of ST, or almost any monad for that matter, leads to serious problems in Scala since you basically have to use trampolines and take a massive performance hit. It's also extremely verbose to do it correctly.

It's a good solution in Haskell though.

[runT1ME]

This is FUD. You often don't need trampolines and there are better solutions than trampolines.

[asdf1234]

It's debatable whether abusing Free is a better solution. Either way, programming like this is disgusting in Scala and comes with tons of caveats compared to Haskell.