First some very general statements.
Usually we don't really know why do we like a game. I can say (suppose) why HOMM V is bad: small number of castles killed the strategy and new battle mechanics killed tactics. But I can not say why HOMM IV is bad. It has many improvements which are very nice by itself. A fraction system, magic system, hero promotion system, battle system in particular were very nice with few minor faults. But overall gameplay was much worse then in HOMM III. I don't really know why.
Second, my position towards Disciples 2: I strongly appreciate its style-atmosphere-art and storyline, but the gameplay for me was slightly too boring. I did not finish expansion campaigns.
Then about KB:L. It has lesser potential and replayability then HOMM III mainly because of absense of random generated games and limited modding possibilities. It's too early to compare campaigns as KB:L expansions are to be made. But the battle system is better then in HOMM III. How do I mark it: a hardcore player have longer way to perfection.
Many may have a pleasure of playing but few can make something like "no losses on impossible". So you are always have a way to become better tactician and strategist. It was not so realy in HOMM III IMHO - the cap of perfection was much lower.
And I really like the the concept of rune-sphere system of hero promotions (as well as its concrete implementation). I think it is the first system of that kind in gaming but I know analogies in the area of sensorics. That is what really can provide flexibility and specificity in the same time - that means great variaty of viable pathways and good adaptation mechanism.
It is hard to compare tactics in KB-HOMM with XCOM-Jagged Alliance. The latter have much bigger and diverse arenas while the former have developed magic system and those make different kinds of tactical systems. Now I don't see the way to combine it successfully. Maybe AD&D gives something intermediate.