Probably assembly would be the main concern. They are generally put together by people that are not skilled in bike mechanics. I've heard of people buying Walmart bikes and then having a bad crash caused by something falling off the bike. You might instead just experience frustration changing gears if the shifters are not adjusted well etc. The biggest indication of what Walmart thinks of bikes? Clicked on the link you sent and bikes are classified under the top level of "Toys".

Also obviously for the price they are selling the bike for, very little human time can have been spent on adjusting the bike. Very different when it comes to buying a second hand bicycle.

Try spending the equivalent amount on a used bicycle preferably from a bike store or a recycle-bike-type coop where someone has put some effort in making sure its mechanically sound. Craigslist is risky unless you can take a friend with you that has some knowledge of what to look for. Best bets are something that has been lovingly cared for by an enthusiast, or something that someone bought a quality bike from a bike store that did a good job of assembling and adjusting, but basically did not ride and stored indoors.

If you test ride the bike, there should be no strange noises, or feeling that something is dragging. The wheels should spin freely and not wobble (try watching them relative to the brake pads, easiest to pick it up and spin them for this). The brakes should stop the bike well and not squeal, and of course the pads should not rub on the wheels while they are not engaged. Try a whole range of gears, shifting should be relatively smooth. Ask the seller if the bike has been in a crash. Inspect the frame does not have any visible dents/damage or rust.

Sometimes a used bicycle may just need trivial repairs (it has a flat tire etc.) but if you can't test ride when you go to look at it its hard to judge how the rest of the bike is doing.