The one we went through got most of their fees via "highly recommended" legal and accounting service kickbacks, which charged far higher than market rate monthly retainers, for doing absolutely nothing in some cases. Lawyers and accountants who stood to benefit were literally partners at the fund... We decided to stick with our own accountant, and the accelerator got extremely upset at us for doing so. Some shockingly unprofessional, threatening, vague emails and lots of criticism for not choosing their accountants.
You are correct that the people running these funds will get paid no matter what, and will have absolutely no financial incentive to make you succeed. Quite the opposite. Ours in particular has done a lot to minimize their work, drag out anything they're required to do over months, and get rid of companies as quick as possible after their investment.
These can still work out, just know that these aren't the tier #1 funds that we're used to hearing about. Founder beware.