With that title line, I am thinking that there are hundreds and hundreds of things they won't tell you, but I will zoom in on a few of my personal favorites, in no particular order:

• Those prices - unbelievably, are way overpriced.

This one reminds me of cosmetics, if you figure out the real price, it is astonishing.  My comparison charts for stabilizers (among other things) is available here.  When I did the comparison, I was careful to select the other sellers at random.  Time after time, AnnTheGran was the lowest and joining Ann's Club made a lot of difference!  In their defense, this is their bread and butter so that they can continue to stay viable as a company.  After all, they are in the business of being profitable.

• Tear Away is only paper.

Tear away stabilizer is, 95% of them, made of paper.  Why on earth do they charge so much for some paper?  Some have a ‘direction' which would create a small difference when you need to use multiple layers.  With the directional types, you would place one at a horizontal position and the second at a vertical position.  That would create a little additional tensile strength (the point at which there is failure) but not a great deal.  Look through your tear away toward a light and you will see, it is likely to be small pieces of paper pressed together.  Tear away is probably where, years ago, some embroiderers decided they could use coffee filters instead of tear away.  Coffee filters are not that cheap, stick with the ‘real thing.'

• You need a quality water soluble stabilizer (wss) for the best results.

Some of the water soluble stabilizers out there should be shot - make that the manufacturers should be shot.  Some are so thin that you can make a hole in them with a single puncture of the needle.  When you place a number of stitches such as a narrow satin stitch, you are going to have failure with a capital F.  For Free Standing Lace (FSL) wss is a must.  I am a big advocate of Badgemaster Water Soluble Stabilizer.  I don't say that lightly, I really mean it.  You can read my Forum post - A Three Year Journey where I describe my process where I got to being a serious fan of Badgemaster.  I am on my second roll of 50 yards by 15" of it and know I won't have problems, even if I have to rework an area.  I also use it on projects where I would not normally use water soluble, heavy density of stitches does not have a negative effect on Badgemaster.

• A professional shop uses just one or two types of stabilizers.

You can check on this if there is an embroidery shop in your area.  They do have machines that are much more precise than we can buy (at a reasonable price), but they don't need 15 different stabilizers around.  What is my point here?  Well, I have about 30 different stabilizers in my stash but I won't use most of them.  I use my favorite tear away, my favorite cut away, my favorite adhesive, and my favorite water soluble.  Those others are still around because I am too cheap to throw them away.  I probably will never use them, but sometimes I want to try something different and any old stabilizer will do. 

What are my favorites?  Honestly, all of them are AnnTheGran products and Badgemaster.  I use them for their quality and price.  Whomever sought out the original marketable brand for this site did their homework on these items.  I do have a 'special use' that is a 'heat away' stabilizer.  Ann does not carry that, but I have found one that works well for me.