M&P 15 Sport - Model 2 - Cheap Quad Rail Install

We install a cheap Amazon quad rail on our Smith & Wesson M&P 15 Sport, model 2!
JTM Drop in Quad Rail:
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Aim Sports Ar15 Delta Ring Wrench Removal Tool:
amzn.to/1Pax5nt
Hand guard removal is quick and easy when you have the correct tools.
Please comment below and tell me what the difference is between these cheap Amazon/Ebay quad rails, and the $100+ quad rails I see in the local gun shops. Not counting free floating rails, what's the major difference in quality?