Wales is still poorer than the rest of the UK and has an inferior education and health system despite having its own government, said the man known as the architect of devolution.
Former Welsh Secretary of State Ron Davies, who led the campaign for devolution 20 years ago, said the Welsh Government has not delivered change.
"It has established now but it must get down to asking itself some very, very difficult questions," he said.
"There are big issues to be addressed."
It was 20 years ago on Monday when the Welsh devolution referendum narrowly backed the creation of the assembly with 50.3% of the vote.
The assembly was given direct law-making powers after another vote in 2011 but Mr Davies, who ran the Welsh Office under Tony Blair's Labour government but is no longer a party member, said the Welsh Government has "big issues to address".
"People have accepted we have a devolved government, the assembly is well rooted in our democratic structure," the former Caerphilly AM and MP told BBC Radio Wales' Sunday Supplement programme.
"And as we see in Brexit negotiations, we see Welsh interests being looked after at the very highest level - so that is a positive outcome of the assembly.
"However, the purpose of that democratic change was to bring about other outcomes - and they will measured by a better health service, better education and economic performance.
"Unfortunately over the last 20 years, even though we have built the structure, we haven't seen these outcomes."
The Yes campaign won the devolution referendum by 6,721 votes - and due to a 50.2% turnout, it was 559,419 of the 2.2m Welsh electorate which voted for devolution.
Rod Richards, the former Conservative leader in the assembly, was a leading No campaigner and said ministers in the Senedd have been "just not up to the mark" since the assembly was formed in 1999.
"Even though I was against it, when it happened I really did want it to work as I didn't want Welsh politics to look like some Mickey Mouse outfit," said the former North Wales Tory AM that has now defected to UKIP.
'Not up to the mark'
Mr Richards said "the assembly has not delivered on public services" because former First Minister Rhodri Morgan wanted "clear red water" between Wales' Labour government and the UK Labour government.
"Tony Blair's government did introduce reforms which benefitted education but in Wales the education policy is determined by a union and schools are still run by the local authority," he added.
"As for economic policy, the big things that needed doing immediately was sort out the Brynglas tunnels, build a proper airport and reform local government and bring the health service and social care together.
"Those are the big things and they haven't been touched because they are too difficult and because the ministers we have had in Cardiff Bay, unlike the ministers we had in the old Welsh Office, are just not up to the mark."
Former Plaid Cymru leader Lord Dafydd Wigley said an assembly representing the Welsh people is better than the "bad old days" of having a "travelling salesman on behalf of the UK government" in Wales.
"There have been successes since we have had full law making powers like the social services and future wellbeing act," the former Caernarfon AM and MP told the BBC.
"The Achilles heel is undoubtedly the economy. We need policies that would engender a greater willingness to set up business and for businesses to have the confidence to expand."
Lord Wigley added: "We haven't taken the burden of local taxation off small business, that is the sort of thing the assembly should be doing, that's where the failure is and until we get the economy right, we're not going to get anything else right".
The Welsh Government has been asked to comment.