National Insurance is an unfair and complicated tax.
It is not paid by large portions of society. (Company directors who choose to pay themselves by dividends, landlords, hedge fund owners, etc.)
The percentage of income paid to NI by well paid employees paying basic rate tax is higher than for high earners.
It is not understood by the majority of the population. The bands, rates and meanings are just too complicated. Most people are unable to calculate or understand the impact of their NI liability.
The money collected from NI seems increasingly to be just viewed a part of the general pool of money available to the government.
The only use National Insurance seems to have been put to lately was when the Labour government decided to increase it so they could claim they'd not violated their "we'll not raise income tax" pledge. It is an income tax in all but name.