It's not necessarily that the ads aren't effective but their reporting of ROI has been off in my experiences. I've been running attribution surveys on the e-commerce sites I own [1] and the results have never matched up with what FB or Google reports back. Some of it may just be bias but I can definitely how some rounding errors may start to creep in with large numbers.

[1] https://www.zigpoll.com/examples/attribution-survey