I am looking at the blog of one of these sites and they proudly announce that “all our students” who applied to Yale made it.
Another one lists out the average admit rate for the college and then the admit rate for their students to the same college which is almost 3x to 4x the average rate.
Can these consultants shape your application in such a way so as to give you an edge or is this a case of selection bias?
Some of these consultants are scary expensive. I can’t come close to affording them but it got me wondering.
Anybody have any experience with this?
Only if you think I could play like Izhak Perlman if I had a Stradivarius violin 
I think they shape their results by pushing kids to apply only to schools where they have a really good shot regardless.
Only hire that consultant on a contingency fee basis with the contingency being that you get an offer of admission from Yale.
Also, what amount constitutes “scary expensive” ?
This is the gimmick. Consultants will definitely help hone and polish an application, but their real expertise is guiding applicants toward colleges that they think the chances are very high.
There’s no magic. Use a reasonably priced consultant to help clean up your application. Don’t use an expensive consultant to help you get into Harvard.
By the way, some high-priced consultants are like elite colleges themselves - they won’t even look at you unless you have very high stats and strong ECs to begin with.