For one thing, it produces atrocious incentives.

I don't understand what this means. Are you implying that an ethical system needs to provide incentives for people to abide by it? The only ethical system that does that is egoism.

If your goal is to help people, the only incentive is people getting helped. If optimal philanthropy helps the most people, it has the best incentives. Doing good is it's own reward, provided that it's what you're actually trying to do, rather than be happy or have a meaningful life or something.

Another huge problem is the creation of unbounded obligations.

There is no upper bound to how much money you could make. As such, the opportunity cost to making any finite amount of money is unbounded. This does not make you infinitely poor. Similarly, being able to do an unbounded amount of good and not doing so is not infinitely bad. Opportunity cost is just a way of reframing the problem.