As you hinted at you really do need to use the right strategy/play style for these generals. If you mainly play the US or China Tank Generals you will initially find it hard to use these Generals.
With both of these Generals the key is to flood the battlefield with cheap units. You need to get as many units as you can to overcome better defended enemy forces. Make sure your units are as spread-out as possible. This makes them less vulnerable to weapons with splash-damage (area-of-effect weapons) which are the biggest threat to these Generals.
Specifically for each General:
The Chinese Inf needs to use larger numbers of it's units to overpower stronger opponents.
The GLA Demolition General hinges on having a good constant flow of demo units to keep pounding away. More like a steady stream of units than a flood.