1) It's complicated. And can really terrify the vulnerable. It may even put them off taking the work. Get a criminal record for benefits fraud as that advert has been on again, the local chavs think they're Dog The Bountyhunter, and you earned 20 quid over the limit and forgot? Truly ridiculous.
2) 20 quid is too low. If you are disabled the work situation is highly variable. And much as there is a reasonable morality in the council taking a bit if you are lucky to earn a few quid this 20 quid limit is too low for this day and age.
Frank Field ( MP ) is doing a Poverty review over the year last I read, so this may be on the cards already. But even so, just to add it here for the sake of completion. If you only earn 70 quid in one month extra, and it's earned in 1 week, you only see 20 quid.
And work for the disabled is just not that predictable. Especially at present with all those cutbacks. As well as being extremely tiring for them.
It costs about 900 quid last I read to do a Benefits Fraud hearing. Over 30 quid too much? So what's the point there as well? Indeedy. It needs a good reviewing, and adjustments and revokings may well need to be made.