International dementia expert Professor June Andrews said the alleged collusion was preventing a public debate on how the health service should cope with understaffing and its lack of resources.
Ms Andrews said failings that patients and families would consider unacceptable are likely to be routine in many hospitals. These include staff being unable to ensure frail patients receive meals or can be escorted to the toilet.
Ms Andrews, director of the Dementia Services Development Centre at Stirling University, said: "Managers and politicians are in a painful situation where they have to collude in an attempt to present to the public a picture of hospitals that is completely unrealistic."
She recently co-led an independent review of two Welsh hospitals that revealed major patient safety lapses and caused the country's government to launch an inquest into the care of elderly patients.
Ms Andrews, who is expected to be named one of the UK's most influential nurses by the Nursing Times today, said a similar effort was needed to find out whether Scotland has similar routine failings.
"A key question is 'how bad are Scottish hospitals?' How do we know it isn't happening here?" she said.
She said there was no reason to expect conditions on general medical wards to be different. "Last week's report on Scotland's NHS continuing care by the Mental Welfare Commission showed that even in parts of the Scottish hospital system which specialise in care of older frail patients, there is something sadly lacking," she said.
The review, Trusted To Care, on the Princess of Wales hospital in Bridgend and Neath Port Talbot hospital, found a catalogue of concerns including patients going without water, food and medicines, or being inadequately washed. Other patients urinated in beds as nurses did not have time to take them to the toilet. The report, published last month, was described as shocking by Welsh health minister, Mark Drakeford, but Ms Andrews said it should not have been, as politicians should know malpractice takes place, because of budget and staff pressures.
She claims problems are systemic and scapegoating staff when problems emerge is unhelpful. "The Scottish Government already knows that there are some problems in hospitals But it is not generally known what the scale of the problem is," she said. "We need a more open discussion about the fact that a hospital is not a safe place."
In many cases, a patient who has been receiving care services at home may have been getting better care than they would get in a hospital, she added.
The public know the crisis in public finances means budgets are tight and there are not enough medical staff, Ms Andrews added. "The logical conclusion would be that people ought to anticipate certain unpleasant and bad things will sometimes if not frequently happen.
"If we admit that we could start to work with the public to improve hospitals. We could ask them what are you going to do about it? For instance families could employ carers to go in to hospital, or volunteer to go in and help with feeding."
Debating the limitations of hospitals is a thankless task for politicians as they know they would lose votes, while NHS staff and managers face career consequences if they raise them, she argues.
She said: "The solution is honesty. Take a deep breath and tell people what is wrong with hospitals and involve them in the solutions."
Professor Jason Leitch, Clinical Director of the Scottish Government's Quality Unit, said: "I do not believe these comments reflect the reality of the NHS, where hard-working staff strive daily to deliver safe and effective care.
"We noted the findings of the Mental Welfare Commission's report on care for dementia patients last month and, as the Health Secretary said, we are committed to implementing the recommendations of this report.
"We are working closely with Alzheimer Scotland to shape our care for those with dementia. This is being supported by an additional £500,000 investment in education and training for front-line staff."
More than 600 dementia "champions" are due to work in hospitals and social care settings in the next two years thanks to increased funding.