Adviser networks had half of the complaints against them referred to Financial Ombudsman Service (FOS) upheld in the first six months of the year.
The latest results from the ombudsman also indicated a sharp increase in the rate of complaints upheld against private health insurers.
Both Sesame (51%) and St James's Place Wealth Management (49%) were shown to have half of complaints against them upheld.
In total, Sesame and St James's Place had 108 and 38 cases respectively brought in front of the ombudsman between 1 January and 30 June.
The rates are slightly down on the last six months of 2009, but remain above the average uphold rate of 44% for all businesses.
Openwork is the only other adviser network to appear in the listings, as the FOS dealt with 49 complaints concerning it, but it's uphold rate reduced noticeably to 21% from 37%.
The private medical sector has also been hit hard over the last six months with some providers seeing complaint uphold rates almost doubling.
Simplyhealth has seen more than three quarters (79%) of complaints against it upheld, nearly twice the figure for the second half of 2009 (43%), although the number of complaints against the provider taken to the FOS dipped to 37, from 47.
AXA PPP (52% up from 34%), Bupa (50% from 26%) and Groupama (42% from 29%) also saw significant increases in rates of complaints upheld, but the figures from Bupa and Groupama include data from other insurance products due to the collection method used by FOS.
Aviva Health was the only medical insurer listed to see its upheld rate drop, from 46% to 38%.
In total the FOS received 2000 more complaints in the first six months of the year than during the second half of 2009.
It received 84,212 new complaints, and of these new cases, 89% related to 160 financial businesses (out of more than 100,000 firms covered by the ombudsman).
Five financial services groups continued to have more than 3,000 complaints each, which together accounted for 47,507 cases - over half of all the new complaints received by the ombudsman during this six-month period.
The number of new complaints about each of these individual businesses ranged from 30 to 12,750.
