Psionic Ascension is the most fun of the Ascension options but I've always thought of it as the weakest mechanically. No guarantees of the boosts you get and also getting random debuffs. Plus the chance of dooming the galaxy (sure, you can say no...but who would?).
Becoming perfect immortal machines is probably the strongest one mechanically, though the traits of genetic ascension are pretty powerful too.
But I'll take interesting over mechanically balanced any day
A perfectly balanced single player game will become perfectly boring very quickly. The goal is what's the most fun. Is one of the reasons I hate that modern RPGs scale enemies levels to yours: a top level character should be a damn near physical god compared to 99% of the world! Otherwise why bother leveling up at all?
As for federations, A-Specs video on it makes it sound like they're taking inspiration from CK2 for some of the mechanics.
Also that they may be waiting for HoI4s Spy update/dlc to land and see how well it works before implementing something similar for Stellaris. Lots of cross-pollination amongst the paradox teams to figure out what mechanically works that they can add the flavour and styling and tweaks to for the different settings.
From what I've heard the mechanics for Dig Sits in Stellaris are the same as for sieges in Imperator. Though I think they work better for something passive in the background, like research, instead of something deliberate and direct like military action. That should be more in the players control. But this isn't the "Wtf went wrong with Imperator" thread xD