It all depends on the incentive structure. This product here is not the typical job agency, but a negotiation service that you can use when you already have an $100.000+ offer. Presumably, they will get paid proportional to the excess they can get for you on top of that, as either money or benefits (otherwise, you would just accept the offer).

The information asymmetry between the employee and company is huge: they know the market inside out, they negociate compensation packages every day and are strongly motivated to hide how bad they need to fill the particular position and how in-demand are those specific experts. Meanwhile, you only know your own salary and expectations, you are unable to objectively rank yourself and go on bits and scraps of information gathered here and there. This is a situation where you are ripe to be taken advantage of, especially on a booming market.

Where by "taken advantage of" I of course mean "sell your product for less than fair market value".