The weight is the main issue.Even an armored car will weight at max 6 kg.Combined with the high top speed,slow acceleration compared to a normal featherweight.The maneuvrablitily is another issue. Car style steering and tank style are vastly different.With car style in the big disadvantage in RW conditions
Also a cheap RC car is in the most cases equipped with a poor quality, interference prone RX set.

So if you want to try this way, get a cheap car, with a replaceable RX set, ,then a good RX set- the cheapest we use in the Netherlands and Belgium with good results is the Graupner X412.

Then you get a sheet of polycarb ot simular,and make the car in an armor like a turtle or the Leonardo DaVinci tank.
Most important issue is then not to fight the better spinners, or if you do, prepare to use a binbag.

But, we have seen the Banger Cars in Brighton, and that was good fun, Unarmored cars going against feathersflippers. Flying meters trough the ar, landing very hard, but when on the wheels, happely driving along.