Jakob is one of the best units in the game, and imo is one of the top 5 units in Conquest. Early on he has amazing offense because base stats as a Paladin and the skills you get from the cav line, shelter allows for really cool strats with Azura dancing, and by partner sealing Wyvern Corrin, Elsie, Beruka, or Camilla he can go Malig Knight for trample and more damage stacking, and flying shelter. If you partner him with Azura you can make him a falco knight, get staff utility back, pick up rally speed, and I prefer to take him to Kinshi after all of that for Darting Blow + Elbow room for his personal offense stacking, rally speed, shelter, and amateratsu for utility.
He never really falls off because of how easy it is to abuse the reclassing system with him and getting skills much earlier and allowing huge stacks before you could even imagine doing them without Jakob.