I have a feeling that the Empire has a few peculiar qualities that make them more susceptible to nerfs, and developers are less eager to buff them because of those qualities. I do not have a sound evidence to back this up, this is all just me theorizing.
Nevertheless, due to these qualities, according to my theory, the Empire got the worse end of the last few releases, and that could go largely unnoticed because of the crutches we had (Palpatine, X7). Now that they are gone, we've got nothing to show, and the powercreep snuck up behind us.
The first of these qualities is the way one of the most prominent Empire lists, the Aces, win. Since they rely on green dice and damage mitigation through tokens, it is entirely possible to win a tough game by not loosing any ships. Its simply the way they play. A 1 health Inquisitor still has access to PTL, the title, and its amazing dial. It doesn't lose anything in offense, and still as much of a threat. Getting an inquisitor to 1 health might have been the result of 3 or 4 good attack rolls, mostly evaded. Still, at the end of the day, the result is 100/0, and the Empire wins. That, if happens many times, looks powerful. People start saying "Man, I can't kill these Imperial aces, they are invulnerable". Now, in reality, those games might have been balanced. I'm just saying that because of the green dice defense, the Empire either wins big, or loses big. And if it wins, it is very very obvious from the final result.
The second quality is the "villainness" of the faction. They are the bad guys, and at the end of the day... they are not supposed to win. I believe FFG is very conscious about releasing Imperial cards and pilots specifically because of this. It's not that they don't want good imperial ships, it's that they really don't want too good imperial ships... Moreso than too good rebel or scum ships.
I mean, to see FFG's reluctance in giving out power to Imperials, there is no need to look further than the K-wing and the Punisher. They are twin releases, supposed to fill the same role, and were likely developed in the same time. I think it is entirely likely that the K-wings turned out to be a bit more powerful than intended, and the Punishers a bit less so. I don't think the designers purposefully made such a bad ship, and I can imagine a bit of oversight on both ends. However, even a blind man can see that the two ships are not supposed to be equals. The K-wing is clearly superior in every way imaginable, in slots, maneuverability and pilot abilities. I mean, can you look at Miranda's ability and think that is not one of the bests in the game? Especially with a PWT.
Now to my next point: Powercreep. It happens in every game, it is unavoidable. Powercreep in Xwing comes in 2 major forms in my opinion, both are well known in the forums as "Red dice creep" and "Dial creep". None of these phenomena is really observable on Imperial ships. Red dice is generally something that we see little of in the Empire, and even if we get a usable ship with 3 red dice, it is rare that we have the upgrade slots to pair them with. The only newer releases with 2+ attack are the Striker and Upsilon, with an honorable mention to SF Tie. All have their attack values justified by their role, and none of them have Fenn Rau like bonuses to attack. (in short, they are not bomb carriers with a 3 primary... looking at you Scurrg)
Dial creep also avoided us, and we only ever see the dark side of it. I think in today's imperial roster, the only ship that could be accused of this is the Tie Adv. Prototype, but then again, I just think the normal TIE advanced is lackluster rather than the Prototype OP.
Finally, the taking away of the crutches. We have relied on Palpatine and X7 a bit, and apparently this have been calculated into the releases we have been getting. The last two waves have been lackluster. The TIE SF promotes a different playstyle for the faction and may be the herald of things to come, but it doesn't feel to me like the grand new thing. It just feels unimperial. The Striker is great and cheap, but the pilot abilities (and their PS) feels like FFG was trying to play it safe. Meanwhile the Protectorate, the Shadowcaster, Sabine's TIE and the ARC-170 have all pretty much became staples of their respective faction.
And now, our crutches are taken away, we are standing here lame. And what shall the next release bring us? A new heavy hitter toolbox with a cheap cost and ALL the upgrades? No. A defensive tank that brings something entirely new into the game? no. What we get is an imperial Y wing. I really hope we get something Great in pilot abilities AND the dial, otherwise I don't think we'll see the imperials coming back for a while.
TL;DR: FFG doesn't want Imperials to be too strong and they have been excluded from the really good stuff in the last few releases, while the other factions got staples and scum even got 2 of their 3 best ships. After the nerf we have very little to rely on.