I previously wrote on the tension between Legalism and Antimonialism.
There is a similar tension between extreme Pietism and Antimonialism.
The legalist forms a codified set of rules which must be obeyed to demonstrate one's salvation.
The pietist has no such set of rules. Rather, they look for trends of outward behavior (usually framed as "separation from the world" or "sanctification").
Of course, this is much worse.
At least you know where you stand with a legalist (you have violated Section 21.36.14b through g, and Sections 131 through 135).
The pietist can simply say, "You haven't improved enough" or "You're of the world".
Everything you enjoy, is "of the world", and must be done away with, "for your spiritual well-being". Any enjoyment of things from before conversion, is "reversion" or, worse, "sin".
The disproof of this stand is easy enough. It focuses on judging our brothers, rather than helping them. Also, it allows for the identification of tares - which is explicitly forbidden by Matthew 13:25-30.
The real irony is that the pietist is no better than his brother. It's a classic case of Matthew 7. Of course, if you try to point this out - or worse, to point out your brother's sin, the pietist has a long list of your "sins" to hold against you - while ignoring his own.