CPA always causes problems.
One party always loses in CPA. There is no win-win.
Either the casino is happy with the deal – and more likely than not this is because they make more money than they would if it was rev share. In that case the affiliate loses.
Or, the affiliate is happy because s/he sends low value players and still gets decent money for them. In that case the casino loses, and soon will boot the affiliate and all kinds of arguments ensue.
Cases where the CPA actually matches the player value are a rare coincidence.
On CPA you will never know when a whale will cross your path, and most of the revenues come from big players. The casinos know this, and count on it.
Since the casino will simply stop paying you if your player value is too low, generally in CPA the casino wins and you lose.