The "problem" with Imperials is simple - the war of attrition is big in the Empire. Spending their starfighters, stormtroopers and pilots against terrorists and Rebels is just standard procedure. Heck, it's actually quite an honor for an Imperial pilot to sacrifice himself in service to the Emperor (Ever get rammed head-on by a TIE Advanced while flying an A-Wing? Yeah, that encounter completely relieved me of my thankfully fully charged forward shields...).
This battle philosophy doesn't gel so well with a tabletop game where a single TIE/ln still costs $15. I think FFG tried to balance for this to make it so that Imperial players didn't have to spend a Death Star's worth of money just to assemble a decent fleet. How successful they were I suppose is debatable, but overall I believe the reason why the Imperials might seem weaker is because I think they were intended to feel different to play. You don't want both sides having the same feel, do you? Unfortunately, asymmetrical game design is rather hard to pull off, from what I've heard.
As for the Assault Gunboat... the reason why we don't have it I believe is quite simple: The ship has already sailed. It's too late.
According to the X-Wing PC game strategy guide, the XG-1 Star Wing assault gunboat was created basically as a hard counter to the success of the flexible T-65 X-Wing. In fact, the Imperials were actually starting to use hit-and-fade attacks similar to the Rebellion's. The XG-1 was actually introduced into service before the events of Episode 4 - specifically, midway through the second X-Wing tour of duty campaign. This campaign ends immediately before Episode 4 starts.
Having both flown and flown against the XG-1, I can understand the sentiment of wanting it in the game - heck, I share the sentiment. I know how it performs; it's a slightly slow dogfighter that has great deployment flexibility. Its armament in X-Wing - shielding, hyperdrive, dual laser cannons, dual ion cannons and dual warhead launchers gives it ideal loadout for a variety of situations. They were usually deployed in flights of three, if I recall. It was used as both a strike craft and a response fighter in lieu of a larger capital ship arriving to launch TIEs.
But it's distinctly possible that the metagame has advanced to the point where the XG-1 would not be competitive anymore - basically, it's not worth creating. First of all, now we already have T-70s, E-Wings, and all the other powerful ships that could and most likely would rip an XG-1 to shreds without really trying very hard. We already have much better ships - the TIE Defender was originally supposed to be an overpowered beast reward for players reaching the last few missions of the TIE Fighter PC game. Yes, the XG-1 was in TIE Fighter as well as a flyable craft, but when everyone has Defenders zipping around on their tabletops, why would you bring an XG-1? I suppose the answer is "the same reason you'd bring a T-65"; filling up a squad. But it's probably easily argued that one could use newer swarm fighters instead. Personally I don't care that it would be "old," since I don't care about competitive play - but I can't blame FFG for deciding to make ships they (gasp!) think will sell.
I would sorely like to see an XG-1 released by FFG. But I honestly don't think they'll do it... they will probably feel the need to release new ships that are competitive with the newer ones, not ones that were designed to fight T-65s and Y-wings.
Now if a new model of the XG-1 shows up in the next movie, on the other hand, that might change things...