Just as the support for earmarks is often bipartisan, the opposition can also be bipartisan. While those on the right often oppose earmarks for promoting wasteful spending or higher levels of spending, those on the left argue that earmarks can divert funding from criteria-based grant programs.
See this example from the environmental group National Resources Defense Council (NRDC):
Though earmarks have important applications in some contexts, they bypass what is supposed to be a merit-based or competitive allocation process under the State Revolving Funds (SRFs), threatening to disrupt access to needed monies by siphoning off most new appropriations to more politically potent projects. An additional problem with earmarks is that they often provide grants (or so-called loans with principal forgiveness) to communities that are not disadvantaged and that would otherwise only be eligible for loans. While true loans to wealthier communities must be repaid (revolving money back into the SRFs to be lent to other communities), earmarking grants to such communities means there’s less funding available to revolve back into the funding stream.
https://www.nrdc.org/experts/erik-d-olson/all-earmarks-harmful-water-policy
Proponents of earmarks would argue that Members of Congress know their districts’ needs better than government bureaucrats, but this also reinforces the point about politically popular projects being selected over the criteria that Congress may have collectively put in place in the law that authorized the program in the first place.