The trouble is, the people that claim to know someone is committing fraud do not know all the facts, and more often have no idea how the benefits system works.
PIP is not means tested, yet there have been people on here go on about their neighbour on PIP who has a nice house and nice car. Or they have seen them go out when they know that this person has said that they struggle to leave the house (people can have good days... you are allowed them). Or their wheelchair using aunt has been seen out dancing at a wedding (ambulatory wheelchair user).
Regarding working on UC. You can actually work on UC. You can earn certain amounts which varies depending on if you have housing costs or not. The cash in hand thing is different, but would you report someone who is not on UC but works cash in hand on the side to avoid paying more NI/tax? Probably not.
And there is a myth that you can't have a partner stay over for more than 3 days a week. I had a Google about this and there is no limit at all. It is more about sharing living costs etc. But some comments on other sites were saying that you can't have anyone stay over at all, and you can't even have a takeaway with them. So how many people are calling up DWP to report the single mum across the road who has her boyfriend round a few times a week to watch a film and share a pizza? All based on myths about the benefits system.
Then people moan "nothing was done". For starters, you wont get told about any investigation or outcome anyway. A Google (AI answer) says that 85% of reports are false. But if you report someone, even if they are doing nothing wrong, then they will have their money stopped and be caused a lot of stress, anxiety and hardship. Some people lose their homes.
There have been people on here who were reported for fraud when they are not even on benefits.