Looking at your link, these people were caught reasonably quickly.
One of the people in your link managed to claim just £231. At his age he'd be getting £73 per week, so he fraudulently claimed less than a month.
It's harder to work out the other person, because they were also claiming housing benefit.
He lived at Ballysillan Crescent, Belfast, which is a B14 post code. Local housing allowance rates are https://www.nihe.gov.uk/index/benefits/lha/current_lha_rates...
Here's that street on Google maps: https://www.google.co.uk/maps/@54.6291805,-5.9663763,3a,75y,...
Let's take the one bedroom rate of £86.16 per week, plus £73.10 per week = £159.26 per week. (this assumes he claims as a single person, and it lowers the weekly amount. I do this because I want to maximise the time he claimed for as a worst case). £4,114 / £159 = = 25 weeks. So, about 6 months. If he was claiming the 2 bedroom rate, or was claiming JSA as a couple he would have got more moeny per week, and so was caught much quicker.
You'll find similar in the other cases. People try it and they usually get caught within a year.
Are few people dishonest, or are most dishonest people caught.
My concern is if the system encourages moral compromise for survival and what this means for our society as a whole.
5s Google - http://www.4ni.co.uk/northern-ireland-news/243195/other-news..., some guy claiming whilst working.
Assuming not all cases are found - and stories I've seen in the past have been of people caught after well over a year of false claims - then it seems perfectly doable for those who are dishonest (through necessity or otherwise).