I played seige when i was first starting playing like a year ago. What i didnt like seige and in part why i went to battle was the balance, not just the player balance, but mainly the map balance. Some maps there might be an 80% chance of attackers winning or even higher and vice versa for defense on some maps. Whether u kept your multiplier depended on if you were lucky enough to get on the right side for that map. Some castles arent really captuarable, and some are so poorly defendable that you cant hold out however long you have on a map. This isnt alwasy true if your team is clan stacked, but this was not a problem when i palyed siege, it didnt seem anyone like any clan was really dominating seige.
But i can definitly see that being a problem, especially given that most of the "randoms" that play on seige are newer players like i was when i started out cause battle is pretty rough for new players. This makes it even easier for clans to str/plate crutch and roll seige servers filld with new palyers, easy multipler. That isnt so easy to do on battle, there are plenty of other good clans that play in mass on battle so teams can be balanced by clans, and there are also plenty of good clan-less randoms that when on the same team ive even seen roll remnant/pro/chaos clan stacked teams quite often.