Stainless Steel has a Real Recruitment option switched on by default. It gives you the appropriate units for the year you're playing. Longbowmen for England don't come until the English Archers event around 1240 AD for example. Each faction has different units and power troughout the span of the game. The ERE (or, Byzantines) are very powerful in Early era. They can train Scholarii in Constantinople and those are murder in SS 6.4. They get good light cavalry, foot archers, decent infantry and decent HAs. An overall very strong faction.

Cav is very strong in SS, especially General's Bodyguard units. Try fighting Eastern GB units in Early with a faction like Hungary or Poland or Siclly and see what I mean. The mod has it's qirks but it's much better than vanilla.