Byz don't get any decent spears IIRC. You are right the novelty of katanks wears off, once the chiv Knights etc become available. Pron A remain useful though and should match the horde. And in Byz cav you have about the best archer cavalry going (Boyars are better but I find the Russians dull.)

VG will eat a cav charge pretty well, especially if you have the upgrades from Sweden. Actually they eat anything then. (Watch out that you can't get them in late.) So will buffed Byz Inf, largely due to being 100 men units, but you need to be building them in a province with morale boosting upgrades. I've done bridge defences fine with Byz inf as the block (using my patented hollow box technique rather than a straight plug, often as not the unit crossing routs without even fighting)

No idea why VG require a spear guild, be thankful that they aren't a merc only unit which would be more historical, I guess its probably game balance to make you build the higher level spear buldings (which otherwise would not give you any inf at all)