Michelle Thomson and Kenneth Gibson both criticised the poor financial planning for the landmark legislation during a meeting of Holyrood’s finance committee on Tuesday.
Ms Thomson said there was a risk of MSPs signing a “blank cheque for the public purse”, while Mr Gibson said it represented a “monumental risk” for taxpayers.
The policy, which will radically change the way care services are accessed and delivered, has been hailed as the biggest reform to public services since the creation of the NHS.
But the legislation has been criticised by council leaders and trade unions, who have said it could lead to “years of disruption” rather than improvements for people in care.
The plans would strip the running of social care services from local councils and hand this responsibility to care boards, which would operate in a similar way to NHS health boards.
This means that the Scottish Government would ultimately be responsible for their performance, which ministers say would improve accountability.
The legislation was produced following a review of the future of adult social care triggered by the Covid pandemic, which put services under unprecedented pressure.
The financial memorandum underpinning the legislation says that the total cost of the initiative could be anywhere between £644m and £1.2bn in the next five years.
Ms Thomson told a Scottish Government official appearing in front of the committee had “no confidence whatsoever” that the plan would be value for money.
“I did look in the financial memorandum specifically on the word risk, and the word risk is only mentioned twice there. But what this is screaming out to me is a huge risk,” she added.
“At the moment from a financial scrutiny [perspective] I’m looking at a blank cheque for the public purse, and I find that deeply worrying.”
Mr Gibson said: “It just seems to me to be a monumental risk to have a bill of this nature, with all the financial implications, because there are a few service deliverers who are not currently up to scratch.”
Responding, National Care Service development director Donna Bell denied taking a “cavalier approach” to the financial planning of the scheme.
She said her staff had spent a “huge amount of time” developing the memorandum, adding: “We’ve talked a lot about the risks and I can provide some real reassurance that as part of the programme work, the risks are very much at the front of our minds, whether those be delivery risks, financial risks.
“So we are absolutely aware and actively managing the risks associated with this programme.”