Professor Andrew Goudie said Labour, the Conservatives and Liberal Democrats faced a "serious question of credibility" over their proposals to extend devolution in the event of a No vote in September's referendum.
His comments came in a paper for the Fraser of Allander Institute think tank published yesterday. All three parties have published proposals for further devolution in recent months. They agree Holyrood should have greater control over tax, including income tax, but their detailed plans differ significantly.
In a bid to reassure voters, who according to polls favour more powers, the three Scottish party leader issued a joint statement earlier this week highlighting their commitment to further devolution following a No vote.
Mr Goudie, now a visiting academic at Strathclyde University, said the prospect of more powers could have a "huge" impact on the referendum debate.
But he added: "It is surprising that the pro-Union group has managed this issue so poorly and in such an incoherent manner.
"The inability to offer a unified view on what enhanced powers might constitute is arguably one sign of this, with the Scottish Labour Party, Scottish Liberal Democrat Party and Scottish Conservative Party all producing their own documents."
He said the failure raised a "serious question of credibility," adding: "Can this stance be trusted to deliver, following a No result? What might this compromise set of new powers entail? Whether this disparate set of constitutional alternatives is understood, or is sufficient to persuade the electorate is, of course, the key issue. Convincing a highly sceptical and suspicious electorate will be a massive task."
In a separate development, Finance Secretary John Swinney accused the No parties of failing to explain how they would address Scotland's economic challenges in the event of independence being rejected.
The Scottish Government has faced claims that its aim of growing the economy to boosting tax revenues by £5 billion by 2030 is not realistic. But in a speech today, Mr Swinney will defend the plans and say: "Those opposed to independence have had nothing to say on how they would address Scotland's economic challenges or seize our opportunities.
"There are no proposals from those opposed to independence to boost Scotland's productivity performance, to bring our investment in research and development up to the level of other European nations, or to create more skilled jobs in Scotland."
* Britain's nuclear capability would be "wrecked" by the SNP Government's desire to remove Trident from Scotland following a Yes vote in September, Sir John Major has insisted.
The former Conservative Prime Minister also rejected as "absurd" the assertion made by First Minister Alex Salmond that the UK parties would renege on their promise to give Holyrood more powers in the event of a No vote.