Believe it or not, most of the time the damage does not come from the substrate, but rather it from the planter. The wrong use of the pincer (grip too hard, angle not right) damages the roots even before the plant goes into the tank.
Alot of people have been using various substrates with not much problems. The thing is roots do not grow very fast suddenly. Growing that fast will run risks but when plants grow at such a slow rate and the soil is basically not moving, the risks are low. And even if it cuts one root or too it doesn't matter.
ADA soil and Seachem substrates can be used as a standalone and it should be so. They contain rich minerals (and nutrients) and there is no need to cover it with another substrate. By buying the soil I presume you want the water altering abilities (pH/KH reducing for ADA, KH buffering for Onyx).
Like you said, ADA and Seachem are "brands" (which is potrayed as "branded"). There are other similar products but some people prefer to go the safer route as these 2 brands have been used successfully by many many aquascapers.
-Calvin
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