Universal credit, the alleged simplification and supposed unification of all benefits, the pet project of Iain Duncan Smith, is in serious trouble. The national rollout, originally intended for this October, has been delayed until next spring and will reach fewer towns. Many observers, myself included, believe this disaster to be inevitable. Why? Because to work efficiently, benefits must, by necessity, be complicated.

I was a welfare rights adviser in the 80s, when an admittedly rickety system usually worked. Unemployment benefit was the basic payment, either income or contribution based, and until 1981, housing costs were paid along with money covering general living costs. Benefits were complex but specific, with all claimants "visited" at home at least once.

Even so, I believe that the previous system was better for both claimants and advisers. Basic payments were further boosted by premiums, such as the additional payment for central heating, intended to encourage recipients to find adequate accommodation.

If claimants were ill, they could submit sick notes to obtain "additional" payments, such as financial assistance to pay for special dietary requirements. If the need was genuine, GPs would issue a note, and the DHSS (as was) would accept or decline payments (one client of mine was refused extra cash for chronic dyspepsia – that is, indigestion. Nice try).

By nature, disability requires different approaches. Some people will be able to work – where employers will hire them – while others won't. The idea that claims for sickness benefits needed frequent assessments, then several costly appeals, was unheard of.

The notion of simplifying, then unifying tax and benefits is not new. Talk in the olden days was of "positive and negative taxation", or a citizen's income, with personal tax allowances issued as payments, and any excess income being taxed. Seems simple, doesn't it? But computers were new, and the "tapers" required to calculate payments were complex and unworkable. This idealistic solution was usually rejected because of people located on the middle income band – an exquisitely detailed red line, nationally amounting to thousands of people with insufficient money to live on – who would earn slightly too much for benefits and this wouldn't "make work pay" for everyone.

Combining the distribution of benefits and collection of tax requires a gargantuan IT system, a system which is reportedly the problem here, just as it was with the ambitious, disastrous and ultimately – abandoned, NHS IT system, which cost £12.7bn. Universal credit also rolls out in a complex world. People could ruin a perfectly fine idea with their lack of online access and IT skills, which is causing further difficulties.

But let's not pretend. Universal credit is intended to reduce the social security budget, not simplify the system, and its numerous flaws are already apparent. Awards to self-employed claimants assume a uniform amount of income, which is unrealistic. Also problematic are free school meals and prescriptions, because eligibility is set at lower levels than at present, and many working claimants could see their income cut.

The reforms are doomed. The current system is more concerned with sanctions and workfare than the efficient administration of payments, with staff numbers declining. So if you stubbornly introduce a new, single benefit trying to cover the needs of a low-income family, including a disabled child, with the main earner in intermittent part-time work, another allergic to wheat so food costs more, I will show you chaos. What's needed isn't a one-size-fits-all benefit, but one tailored to fit individual needs, not to cut everyone down to the same size.

There is a lesson here for IDS, but also for Labour, which supports the philosophy of these so-called reforms. While the notion of one, simple, universal benefit is appealing, remember this – effective, systematic simplicity is, in reality extremely complicated.