You are correct that they got the power level just right and that they deserve major kudos for that, that can't have been easy 👍🏻

Another compliment they deserve is that they were a very good way to represent all planes (together with a couple of other cards from each plane) truly making MOM feel like it spanned all the planes.

But overall I'm not too fond of them.

Flavor-wise the sieges are weird. So you play a card named Invasion of Segovia (for example). What would you think such a card would represent? It represents… the Segovians fighting back at the Phyrexians—both sides of the card are flavored as Segovians doing Segovian stuff vs them.

OK, so it's called a "siege". Like Helm's Deep. And it's a pro-Segovia, anti-Phyrexia card. So you'd think you'd want to defend it and defend it and defend it, like the Segovians in the story defended agains the Phyrexians. Siege. Simple. But no! You are trying to attack it down to release the sea tyrant Caetus who'll help you fight the Phyrexians.

So when your opponent plays a Siege, that means you are suddenly forced to represent someone who is invading Segovia (Phyrexians, presumably, whether or not you'd ever put any Phyrexian cards in your deck) and trying to defend the—not defend Segovia, but defend the invading force so they can't release the sea tyrant or whatever.