I've been part of r/EthTrader that is one of the subreddits that has this feature and had it for about 2 years now.

Dynamics within subreddit members got so bad that the subreddit split, and now almost all the action happens in r/EthFinance that does not have any community currency. The main deterioration was actually corruption. Because "karma" had monetary values and "karma monetary policy" could be set by admins, there was massive concentration of wealth for mods, with no incentive to change the rules.