Some more specific reasons to not buy a Wal-Mart/Canadian Tire bicycle, having seen many of them (most of these will apply, if not all, to a $99 bike):
-clunky drivetrain components that will not stay in adjustment, including a derailleur attached by a flimsy adapter claw rather than a hanger
-one-piece cranks, or three-piece cranks with large portions made of plastic
-nutted, rather than quick release, wheels
-brakes that DO NOT STOP YOU, often paper-thin stamped calipers or V-brakes (compare to forged calipers or fancier V-brakes; stiffness and good pads are required)
-wheels poorly built, at low tension, with soft nipples and plain, rust-prone steel spokes
-shifters are usually abominations (low-end grip shifters are very difficult to service, and make cables hard to change)
-low quality threaded forks and headests (I like *nice* threaded forks, and use them myself, but they are no longer widely available)
-bearings, bolts, and the like installed to incorrect preload or torque, without adequate lubrication
-frame made of more high-tensile steel or low-grade aluminum than most small cars, with a tendency to transmit bumps directly to the rider (a higher quality CrMo or other nice steel frame will absorb bumps instead)
-often flimsy, heavy, squishy, limp, ugly rear suspension mechanisms, which tend to break quickly

Of these, the wheels and brakes are the most dangerous. Everything else is just annoying. See if you can find a used '80s or '90s rigid MTB in the same price range, with nicer components. I frequently ride an '83 Norco Bigfoot and an '85 Bianchi Grizzly.