Warning: Wall of text incoming. I've provided one-line-summaries in between and a conclusion an the end for you TL;DR-folks out there.
First off, let me emphasize that this post not about game balance issues. By "game balance" I mean anything directly related to winning or losing encounters. What I am going to talk about is "meta balance", i.e. how balanced things are outside of the actual encounters: overall fun, replayability, customizability, difficulty and other things of that nature. A game can be perfectly balanced in the traditional sense and still be horribly tilted in other ways.
Also, let me briefly summarize where I come from so you put things into perspective. I've never played 1E, and I've only played a couple of skirmishes and one campaign up to (but not including) the finale, all of which as the overlord. So my record of 2E games isn't particularly impressive: I've not yet seen all heroes, or quests or items in play (let alone the endless possible combinations thereof). That's the main reason why I don't want to comment on games balance issues. That being said, I don't think the points I am about to make suffer too much from my lack of experience with Descent 2E.
As we moved through the campaign, the hero players were having more and more fun with each quest while my enjoyment remained fairly constant.
This is not to say that I didn't have fun, because I did. The point is that over the duration of the campaign the players were enjoying the game more than I was (in relative terms). Having let several days pass to let the experience sink in, I think I've identified three main reasons why that is.
Lack of Progression
After every quest, the heroes get XP, gold and the opportunity to buy new equipment. With the option to sell old stuff and the chance to draw the treasure chest when doing a search, it feels like the heroes get noticably stronger in each encounter. And most if not all of their new stuff can be used each and every round.
The Overlord, on the other hand, gets one new card, if that. And even then, as powerful as that card may potentially be, I may not even get to see it, or a hero simply passes the required attribute test, or the situation that allows the card to be triggered may never occur, or the surge required by that card may get cancelled by a hero. For all of these reasons, getting a new Overlord card generally feels very underwhelming in comparison.
Yes, occasionally I may get lucky and win a relic. Except that, if I remember the rules correctly, I can only use it when a lieutenant is in play and he may only equip one at a time. It's a nice bonus, but nothing I would get excited about. Besides, these things are not gonna change the next time I play and have no variety and element of randomness like the shop item cards do.
Again, this isn't about balance, I am not saying I want more cards or anything along those lines. It's about having fun and getting new toys. Ignoring the peculiarities of each quest, one plays very much like the other in terms of what the overlord can do.
Why can't I react to how the heroes are being played by my friends? Disregarding balance issues for the sake of the argument, why can't I equip one monster with a huge shield to block ranged attacks if I am having trouble dealing with several ranged heroes? Or why can't I spend two search tokens the heroes didn't pick up on a new spell for my Flesh Moulders that allows them to open doors from three spaces away via some sort of telekinesis?
I guess what I am really missing is the option to customize my monsters, even if just very slightly. As it stands, I have no way of reacting to what the players are developing their characters to. Which brings me to my next point.
TL;DR: I want more toys and customizability. Why do heroes get all the fun?
Loss of Options
During the first couple of quests, chosing monsters for the open group(s) was really fun. Too many melee heroes? Let's go ranged! Primarily casters and guys with range? Let's get hard-hitting melee monsters plus something that can immobilize! That was very cool.
Later, though, that changed dramatically. Eventually, every hero had at least one melee and one ranged weapon which they could equip as needed. Admittedly, it generally wasn't a good idea for the tank to unequip his shield for a bow, but even having the option gave the heroes a huge boost in strategies while at the same time severly crippling mine. Oh, you are gonna use those Elementals and Merriods? Good thing our healer can dispell immobilize without even using up an action. And then the heroes get equipment that allows them to move, stun and immobilize my monsters and there is nothing I can do about it. This is a complete one-eighty from when the campaign started.
TL;DR: As time goes by, more strategies should become available to both parties. The opposite is the case for the OL.
One Mind vs. Four
Being the evil overlord tickles my pickle. I find it very challenging and much less forgiving if you're not paying close attention. Forgetting to move a creature or not reinforcing when you could have can instantly lose you the game. That was at times extremely frustrating, but at least I knew when and how I screwed up and can only blame myself for it.
As for the heroes, they have a much easier job in that regard. There's up to four minds working together on the same problem. Each of them paying attention not to miss anything important. When someone comes up with a plan, everyone chips in with their ideas and stupid suggestions are immediately vetoed.
When I'm about to make a decision, there's noone to double-check what I'm doing. I've to go through various ideas, pay attention to what cards I have in my hand, what cards are still in my deck, what abilities and feats the heroes have (and which of those are exhausted or unavailable due to a lack of fatigue), in which order I should activate my monsters, that certain cards need to be played before an attack roll while others may be played after and so on and so forth.
And all of that is okay I guess because it adds to the challenge. I'm not exceedingly happy about it, but I'm fairly confident that things will become easier the more I play the game. The point is: as the campaign progresses and each hero gains more skills and abilities, the difficulty of keeping track of everything important is amplified by the number of players, while it is the exact opposite for the heroes.
TL;DR: For the Overlord, the overall complexity increases much more steeply with the duration of the campaign and the number of players than it does for the heroes.
I enjoy being the evil overlord. I enjoy playing against three or four opponents at the same time. To me, that is fun. What I don't think is fun is seeing how the heroes get reward after reward - some of it hard earned, some of it through sheer luck - while I sit there and can only choose between the same ten-or-so lackluster cards as last week.
The heroes can adapt their strategy to my cards much more easily and efficiently than I can react to theirs. Playing the same handful of cards over and over and over again gets boring very quickly, even if the encounter itself is very exciting. I want more toys!
How do you fellow Overlords cope with this? Or do you not mind at all?