The Discoverer class needs some adjustments, almost every one of their bonuses is countered by either a negative or a very specific zone of use.
Let's look at the different class bonuses:
+25% mine production - Good at ALL points in the game, all players build mines and upgrade
+10% energy production - Good at ALL points in the game, less powerplats/sats need to be built to run mines freeing up fields
+100% speed for Transporters - Good at ALL points in the game. Lets Collectors both transport goods and farm planets with 2x Speed
+25% cargo bay for Transporters - Good at ALL points in the game, see previous point but they need less ships which means less Deuterium spent
+2 fleet slots - Good at ALL points other than immediate account creation. More standard fleet slots means more farming/trading.
+2 offers - This one is debatable. The market is not really being used properly from what I've seen and heard from other players. This might end up Red or Green depending on how the market works out.
+100% speed for combat ships - Good at ALL points in the game. Faster attacks = more successful attacks since the target has less time to react.
+100% speed for Recyclers - Good at MOST points in the game. Timing your recyclers isn't as imperative with this bonus since your only REAL competition for a floating field will be other Generals.
-25% deuterium consumption for Recyclers - Good at MOST points in the game. See point above except it increases your profit margin for an attack
A small chance to immediately destroy a Deathstar once in a battle using a light fighter. - Negligible at best. A good player doesn't launch on a Deathstar unless they were already going to destroy it. This will just soften losses on an attacker. Defenders might get a bit lucky, but this will more than likely NEVER swing a fight in either direction.
+2 combat research levels - Good at ALL points in the game. +2 to each tech is amazing easily allows an already combat oriented class to excel even farther. In the early game you can't be touched by others players except Generals. In Mid/Late Game it'll significantly reduce your losses and mean you can out tech most non-General classes.
-25% research time - Good at SOME points in the game. Doesn't matter if you can finish a tech 25% faster if you can't afford to start the next research after. And until Late Game, when research starts taking multiple days, this will save a few minutes to a few hours at best.
+400% gain on successful expeditions - Okay at Late/VERY Late game. Early game you don't have the ships to risk sending on Expeditions. At Mid/Late game you don't want to eat up fleet slots that could be farming planets / attacking players for a CHANCE at some loot. The RNG aspect of this bonuses eliminates most of the reward.
+10% larger planets on colonization - Good at LATE game. Early/Mid game fields aren't an issue. It's not until much much later that this bonus will be felt. Also since this bonus does NOT apply retroactively, you'd have to abandon and recolonize in order to gain the bonus.
Debris fields created on expeditions will be visible in the Galaxy view. - Negligible. People have to be sending expeditions for fields to appear. Even then, OTHER CLASSES can also harvest these fields even if they can't see them. Generals, for instance, can buy Pathfinders off the Marketplace and launch on any fields reported to them by another player (or blindly) WITH THE 100% BONUS COMBAT SPEED.
+2 expeditions - Okay at Super Late Game where you have nothing better to do. Since these are simply a +2 maximum expedition slots, you're still tied down by the BASE numbers of fleet slots you have. Most players aren't going to want to send 5-7 Expeditions for a dice roll of loot when those slots could be used for active farming / attacking. You also need the extra ships to cover the +2 Expedition slots if you wanted to use them.
+20% phalanx range - Okay at Late Game. This will probably amount to +10-12 range for most players. But requires a moon and spending hundreds of millions of res on Lunar Bases and Phalanx, not to mention the build time, before you'll see any real benefit from this.
75% loot from inactive players - GOOD at Mid/Late game, EXCEPT that you get countered by the other classes. Both General (using Pathfinders from the marketplace) and Collector (bonus Cargo speed) can out fly you to targets. The extra loot is negligible if all/most of the targets are empty when you arrive.