I never play Empire or Napoleon with the difficulty above 'Normal', for the reasons stated- 5 full armies can't defeat one understrength one anything above that. Even on the lower settings you can get your butt handed to you by a smaller army if you autoresolve, but I usually fight out my battles and can manage pretty well, especially when I have a technological or discipline advantage- I've defeated a full Polish army in Empire (20 units, all at full strength) with 8 Prussian Infantry battalions (understrength) and one generals bodyguard unit. Took serious casualties doing it (I had to disband one of the battalions afterwards and lost most of the bodyguard unit), but I drove off the poles and held my position after almost an hour.
My main complaint in both games is that the Cavalry is totally fubared in both- nearly all units by this time carried carbines (including rifled carbines) and were trained to use them from the saddle, but for the most part only Dragoons and Light Dragoons have them, and Dragoons have to dismount. TOTALLY wrong. Curassiers, Lancers, Uhlans, Hussars et al were almost fully equipped with carbines from about the time of the Seven Years War onward, and were fully trained to use them- even Lancers and Uhlans were set up so that only the first rank or two were actually equipped with lances, the rest had carbines. Most units even had their own 'sharpshooters' equipped with rifled carbines from about 1750 onwards! But you'd never know it from the TW setup.