Correction. FFP is very effective against the regular clubs. As we've seen with football leaks FIFA are fully willing to bend over backwards for their "favoured" clubs.
Just look at how they went at Milan. It certainly is effective when they want it to be.
The problem with FFP is that, as with all financial legislation, avoidance rather than evasion is easy if you have the right people working at it and you know the letter of the law.
As quick as they change the law to catch up the clubs in question will change their policy to avoid it.
Institutional corruption from the top down within the governing body doesn't really help either.