A review of further education provision in higher education institutions said that their success rates had not kept pace with the general improvement among colleges.
Inspectors examined four HE institutions that took over college provision as part of a wider review of FE in HE.
One was inadequate, five years after it took over a failing college, while another had not addressed "a wide range of problems" inherited from the former institutions.
The others remained satisfactory 10 years on from the merger - although they said that one has since transferred its provision back into the FE sector.
Inspectors concluded: "The mergers of higher education institutions with satisfactory or failing FE colleges that had a broad academic and vocational curriculum have not led to significant improvement in the quality of the FE programmes."
At weak institutions, teachers were criticised for poor planning and low expectations of students, while efforts to improve were hampered by unclear lines of accountability across institutions where FE was often just a small part of their work.
The report will provide food for thought for institutions such as Southampton City College, which announced earlier this year that it was considering a merger with Southampton Solent University following its approach to the college.
Lindsey Noble, the college principal, said options from joint projects to a full merger were under consideration, although she expected the college name to remain under any "ownership change".
A merger between the University of Derby and South East Derbyshire College was called off last year when the then Department for Innovation, Universities and Skills queried the deal.