The Law Society of Scotland said "no clear case" had been made in the existing proposals for the Scottish Tribunals Service and Scottish Court Service to be brought together into one organisation.
Money Advice Scotland, meanwhile, voiced concern that the "adversarial" culture of the court system could put off claimants pursuing tribunals.
Tribunal hearings cover appeals over council tax reductions, pensions entitlement, housing issues and compulsory treatment orders for mental health patients.
The concerns came in response to a Scottish Government consultation on the changes.
Legal Affairs Minister Roseanna Cunningham said they would "strengthen" the tribunals system by treating users "fairly, timeously and with respect".
The Law Society of Scotland said it was not opposed to a merger "in principle ... providing that the distinctive character and the integrity of tribunals is preserved".
However, it added: "Having reviewed and considered the proposals as outlined in the consultation document, we would suggest that there is no clear case made out for a merger between the STS and the SCS."
Money Advice Scotland, an umbrella organisation of groups supporting consumers, said it understood the rationale behind the proposal and recognised the "potential efficiencies and economies of scale".
But it added: "We are concerned that the more adversarial culture of the courts could impact on how tribunals are run.
"There is considerable evidence that the public perceive the courts as intimidating, formal and complex and that this plays a role in deterring them from going to court.
"Any shift in the approach taken to tribunals towards that taken by the courts could deter both those with disputes and non-lawyer representatives from pursuing cases.
"It is important that the distinctive and less formal character of tribunals is preserved and protected, ensuring that they are specialist, investigative and inquisitorial rather than generalist and adversarial."
The tribunals service deals with more than 7000 cases a year and has 90 staff in three main bases. The High Court, Sheriff Court, Justice of the Peace Court and Office of the Public Guardian deals with tens of thousands of cases and has 1500 staff at 70 sites.
The Scottish Parliament is already considering a Bill to create a two-tier system for tribunals and appeals, under the judicial leadership of the Lord President. The next stage of reform outlined in the consultation would extend that to cover administrative support for both the courts and tribunals services.
Glasgow City Council said there were fears the tribunal service would lose out as a result.
It said: "Given that the Scottish Tribunal Service is the smaller of the two organisations the concern would be that over time the characteristics of the tribunal system could be eroded to become more like courts.
"Staff training highlighting the difference (and the importance of this difference) between the tribunal and the more formal courts would be required."
Citizens Advice Scotland recommended an independent review of the merged service after three years.
A Scottish Government spokeswoman said: "The consultation on this issue closed last month and the Scottish Government is still considering the responses to this. We will publish our formal response in due course."