@AdamNichol, did you read the bit in the article about losing the work allowance in the subsequent month, because the UC system doesn't "see" any income in that assessment period?
These people know their average / typical / annual 'income'. Just because the government gives them £1800 doesn't mean they need to spend it all and be broke a few weeks later.
The fact that "these people" are on UC at all means they are skint and on the margins financially. There is no "slack" to take up.
Say someone's income is split 50/50 between wages and UC. They might pay their rent out of their wages, and that will be that money gone. They then use their UC to live on for the month. Or vice versa.
Then, one month pay day falls early because the usual day is a weekend or a bank holiday. They still have to pay their rent out of that wages payment, so that money's gone. Then the UC doesn't arrive, leaving them with no money for food, fares, fuel etc.
The following month, when they've probably run up an overdraft or borrowed from friends/family to feed their kids, UC thinks they haven't worked, because no wages have been paid in that "UC month", so their UC is less than it would normally be. And they have debts to pay or an overdraft to clear.
It's an absolute cock-up. The software should have been written in such a way that the claimant's pay period is recorded and, if the normal pay day is within a few days of the UC assessment date, treated as being paid on the normal pay day if/when that date falls on a Sat/Sun/BH.
I'd advise all UC claimants who are being paid early because of Christmas to check how this will affect their UC, because a lot of people will be affected and January is a long month.