Knights are best in the open where they can roll over foot units. 2APs gets you a fast moving unit with +3 vs foot, +4 vs mounted and a handful of quick-kills against various foot units in the open. They don’t do very well at all in bad going. They autpmatically persue so its very hard to keep them under control once combat starts, you also want to keep them away from behemoth which rip them to shreds. I like to have these supported by a paladin or hero, which is fast enough to keep pace and provides some protection against enemy behemoth.
This was a unit type I wanted the good guys to have a truckload of, despite the models being hard to come by. There are a few scenes from the Lord of the Rings films where a wave of Rohirrim Knights smash through ten times their number of foot troops, that was the kind of game I wanted to play.
The only models I could get which were suitable were the Battlemasters knights for both the good guys and the bad guys