PROS & CONS of Using Professional Employer Orgs (PEO)

I'm helping some small companies set up their infrastructure and ramp up operations, sales, etc. These companies are hiring bulk of their staff in one location, but they also have sales staff spread throughout the US and some small pockets of R&D in other locations. Setting up payroll and benefits under the company is a lot of work (filing in each state, payroll filings by state, negotiating small company benefits coverage in today's ACA climate, cutting payroll by state, etc). So I'm considering a PEO like Trinet or Insperity (formerly Administaff). I know they employ my staff under their legal entity and it's a co-employment set up. From what I see, the benefits are that I don't need to file in multiple states, transact payroll and withhold by state, set up benefits plans by state or region, go get Employ Practices Liability Insur, get Workers Comp insur by state, etc. Their fees are fairly steep, but if I only have 15-20 staff, they save me a lot of hours and avoiding errors. And I notice that although their health insurance plans are not always with the best carriers, the premiums that they offer are sometimes 20-30% lower than what I can negotiate as a small company where the carrier will age band my premiums. This sounds too good to be true. What downsides and risks do I need to be aware of? Any thoughts are appreciated.