ROI is insane. If you made 20% a year on Wall Street, you are crushing it. You can make 20% a month in sports gambling with a fairly pedestrian strategy.

The only issue is that you can't scale well but this only becomes an issue when you need to put down tens of million a week (i.e. when you have $50m+).

The barriers to entry are also getting higher as competition is increasing in the space (although limits are probably going up too). One big issue right now is data. If you bet on soccer (the sport with the highest limits), you need to spend at least $100-200k/year (before hiring programmers) on data to become competitive.