“Brains aren’t everything in politics,” said the former Conservative MP Paul Goodman of Iain Duncan Smith, in an article in which he admits to admiring the former party leader and describes him as a “latter day Wilberforce” – comparing Duncan Smith’s welfare reforms to the Slave Trade Act of 1807.

Great reformers, however, must be both conceivers of radical policy and implementers of it. The success of any reform can only be judged in actions, not thoughts. Had William Wilberforce been anything like IDS, I am quite certain the abolition of slavery would still be being tested in regions of the UK with a rollout expected soon. Because, while I could put forward a decent argument on why Duncan Smith is not a great conceiver, I can put forward an irrefutable one that he is a hopeless implementer.

One thing sets universal credit apart as a government policy: it enjoys – and has always enjoyed – cross-party support. Partisanship, therefore, cannot be said to have played a part in its disastrous implementation. Many suggest the project was doomed from the start. “The IT changes that would be necessary to deliver a more integrated system would not constitute a major IT project”, stated 21st Century Welfare, the 2010 green paper that heralded its arrival. By November that year, however, the shadow minister for employment, Stephen Timms, was already warning Duncan Smith of “a serious risk that it will not be ready for new applicants by October 2013”, the white paper’s deadline. By 2011 Duncan Smith was promising that a million claimants would have converted to the system by 2014.

Confronted on BBC Radio 4’s Today programme on Tuesday by the reality that a fraction of that number has materialised (just 20,000, claimed Mishal Husain; 40,000, corrected Duncan Smith), he was in full fingers-in-ears-lalala-not-listening mode. He claimed he had made the decision to implement it more slowly two years ago in order to do it “safely”. Despite assuring parliament up until March last year that everything was “proceeding exactly in accordance with plans”; despite widespread reports that implementation, even on this tiny scale, has been beset by problems on a farcical scale.

“I know you want to try and look at this in an utterly negative way”, Duncan Smith complained about Husain’s line of questioning. As if the Office for Budget Responsibility adjusting its estimate of people on the scheme for 2014-15 from 1.7 million to a rounded down 0 (yes, zero) and the cross-party public accounts committee saying that “much of the £425m spent so far may have to be written off due to poor management” are the results of some complex conspiracy, rather than gross project leadership incompetence.

The question is why is Duncan Smith still in his job? His lack of practical skills can hardly have come as a surprise. “Mr Duncan Smith is incompetent and must go” was the headline of an opinion piece penned in 2003 by his own, recently resigned director of strategy Dominic Cummings, back when IDS was leader of the Tories. Duncan Smith, Cummings claims, is “the symptom rather than cause of a party desperately short of the political essentials: understanding, talent, will and adaptation”.

Ten years later, it would appear that little has changed. Political expedience seems to be the only thought process behind Conservative cabinet appointments, with suitability and effectiveness low on the list. Certain ministers, such as Duncan Smith, are in their job because they would be dangerous on the backbenches. Others have been shifted out because they became toxic to the party’s image. For instance, regardless of what one thinks of their policies, there can be little doubt that Michael Gove and Andrew Lansley went about implementing them with ruthless speed. They found themselves replaced by the largely inert Jeremy Hunt and Nicky Morgan.

This is the Tories’ achilles heel. For rival parties, this failure to deliver is as important as ideological disagreement. One can argue the morality of their decisions, and that will strongly appeal to a section of the electorate. But there is another segment who, when it comes to ethics, frankly don’t care enough. This is often the same demographic that cares about efficiency. There is, in other words, a conservative as well as a liberal case against them – a point made in relation to Duncan Smith by Nick Cohen in the Spectator. To point out that the Tories are “the nasty party” hardly makes a dent in their image. They have long been perceived as such. But voters expect them to at least be competent managers.

Delivery is a more absolute measure, especially if you use their own yardstick to beat them with. Beyond arguing the wisdom of austerity, there is the hard fact that George Osborne promised to eliminate the deficit by 2015, but instead will just about halve it, even using dubious accounting. Beyond arguing about the lack of environmental policy, there is the hard fact of appallingly managed flood assistance and Owen Paterson blaming the badgers for the failure of his ill-advised cull. Beyond arguing about the cruelty of welfare reforms, Duncan Smith promised to have a million people on universal credit by April 2014 and has managed 40,000.

These are irrefutable, merciless facts that add up to the compelling argument that, even taken on its own terms, even being charitable by accepting its own figures, this government has been hugely wasteful and utterly useless.