The 'fairness' is selective though swing.
As well as the complications that helena has mentioned, to claim carer's allowance you have to care for 35+ a week and you get £64.60 per week. For 35 hours (equivalent of a full time job), that's just £1.85 per hour. Plus the random hours, getting up at night maybe etc.
If you have to care for 70 hours (only 10 hours a day so not unexpected for some disabilities) you don't have time or energy for a paid job as well, or the option to claim a double allowance, or to claim another allowance for a another carer to do the 35-70 hours.
That's clearly not fair.
Plus the other changes that kick in at pension age eg changes to PIP vs Attendance Allowance and this can affect people's budgets.
There are many more examples.
So it might seem as though the PC change are correcting an 'unfairness' in the single vs couple rate of PC, but what it shows up is the unfairness and inadequacy of other benefits/ support. And if the UC system was interested in fairness rather than just cuts, there are so many things that could have been improved, but yet all we have seen throughout the new regs is (the most vulnerable) people losing money.