The backlogs of reviews and appeals leave thousands of taxpayers with uncertainty after two years of reassessments that resulted in tax hikes for hundreds of thousands of homeowners. The rising assessments also have been a source of contention between City Council, which commissioned an independent audit that was critical of OPA, and Mayor Jim Kenney’s administration, which has defended the city’s assessment practices but has committed to making improvements and has given in to calls to search for a new chief assessor.