Buncles: Kemious, the reason that buncles are strong isn't that their cards are solid when working together--it's that they're completely numerically through-the-roof broken.
Jade buncle is a level 4 no matter which way you spin it. Either, you have:
45 HP, 40 damage, 4 agility, 2 range (rangeless on that first at+15 hit), 0 DF, which is 14.5, which when compared with Urgrant (50 HP 40 damage 4 agi 1 range 0 DF), is actually above that.
45 HP 10 DF 25 damage 4 agility 1 range. If he's level 3 without the 10 DF, he's most certainly level 4 with it.
45 HP 5 DF 3 agility 3 range and 10 attack--which puts her at 12.5 points, when a level 3 is 12 points, and a level 4 is 14 points. With another 15 attack, she's a level 4 unit. And then of course, she generates SP on open.
Obsidian carbuncle: compared to any great spirit, he's above and beyond ridiculous. The open on its own would have been plenty if you consider the great spirits as balancing factors (EG no level 1 unit open should be better than a GS), but factor in the close, and this thing is obnoxiously good.
Diamond buncle: 25 atk 35 HP 5 df 2 agi 2 range. 11 points already. So once he gets an additional 1 agility, he's a level 3. Usually, he gets an additional 3-4.
Ruby General: buncles basically needed this guy because they did crap damage without him prior to now. But thanks to Amethyst Eater, Obsidian open buff (10 atk) and Agate Carbuncle, that's nowhere near the case anymore. Now he basically one-shots anything with a rangeless attack, and is affected by his own buff and heal, and IMO is on par with the old EXLap.
It isn't so much that "oh, buncles just work together well". Quite a few of them are individually numerically above the curve just in terms of raw stats before you take their abilities into account--which are useful pretty much always (attack reductions, DF buffs, heals, front row free DF destruction). They're about as fast as shrine knights, and definitely a lot more durable out of the box (I suppose if Elaice gets going for a few turns, that stops being the case).
As for Gowen's "tech" mechanics, Kemious, they really aren't tech. They're just inefficient ways of doing damage. The whole focus mechanic is A) not unique to Gowen B) a recipe for mediocrity--either your opponent has no answer, or he gets steamrolled. And most files have an answer for long enough that it's nowhere near as amazing as say, Lawtia's SP denial mechanics, Falkow's manipulation mechanics, or Refess's buffs-out-the-wazoo mechanics.
As for Gowen being "more damage just isn't cutting it", that's not it. It's that Gowen's damage is no longer all that amazing to begin with. Yes, back in sets 5-8, Gowen was the sphere of smash-you-with-more-damage, because the other spheres just didn't have that much damage without having glaring weaknesses. But now, every sphere besides Falkow has very viable ways of hitting you for a lot of damage. And in the meantime, Gowen got nerfed on everything that A) did massive damage (Bringer, Rhino, Dilate, Pixy dual wielder, Mitzette) B) its best tech stuff (RFA SS, pixie locks, wandering samurai) got nerfed to the point that, say, just chain-locking your opponent out of his units with chaining archer pixie is no longer valid. Hitting double RFA with one miracle fruit now costs SP that Gowen can't really generate easily.
So while the other three spheres (especially Refess) got a whole bunch of new options to do things besides damage, the things Gowen had that did its signature damage at good speed got nerfed (Bringer, Dilate, Rhino, Pixy dual-wielder) and best tech options got severely hamstrung, and all it received ever since it got Bringer was set 10 Dilate (nerfed), Jack (nerfed), the pixies (now nerfed), and a bunch of new units that simply tried to do damage in novel ways.
That's just negligence on a massive scale.