Are called shots actually as powerful as designers believedd them to be?

I have been searching for interesting rules systems to make Pathfinder Combat more interesting in an E6 campaign, more cinematic, and to make classes relying on mundane attacks a bit more powerful. Called Shots seemed like exactly what I needed. Indeed, the effects they can inflict are sometimes actually crippling.

However, I was shocked by the investments required to make use of this mechanic.

  1. Like many mundane mechanics in Pathfinder, it requires two feats to function properly: Improved Called Shots and Greater Called Shots. Yes, Called Shots offer a greater array of available effects than, say, combat maneuvers do, but two feats is still a huge investment. Even for an E6 campaign.
  2. Those feats require Combat Expertise as a feat tax, so Called Shots actually cost three feats to work properly.
  3. Those feats require Int 13, which is not always the best choice for a martial character and doesn’t really make a lot of sense for an aiming feat (Dex 13 would make a lot more sense).
  4. There is a big opportunity cost in making a Called Shot even if you have both related feats. When you decide to make a Called Shot, you accept a huge penalty to your attack roll if you want to achieve anything significant (“Challenging” shots), quite likely just missing your target and achieving nothing at all.
  5. “Easy” shots, on the other hand, don’t threaten your opponent too much.
  6. However, speaking about crits, they are unreliable. Even when you build your character around crits.
  7. Debilitating blows’ effects are mostly amazing, but I plan to use those rules in E6.
  8. Most significant effects achieved by Called Shots allow for a saving throw to partially or fully negate them, to make the feat even more unreliable. At this level, 40 points of damage from a single attack are likely to guarantee this creature dropped within a turn or two, even without any additional debuffs.
  9. Apparently, the designers wanted those rules to be unreliable, because the standard rules make True Strike turn your Called Shot into a normal attack.
  10. If you target touch AC, you lose this benefit when using Called Shots, so Gunslingers and magic users stop being as accurate as they usually are.

In my games, I want to buff Called Shots significantly because they seem to be useless, those feats being trap options. The only possible way to use them now is to take Improved Called Shots, take an Easy Called Shot every time you make a full attack, and just hope for the best (the penalty from the shot will be offset by the bonus from a feat). However, I am not an expert in Pathfinder, so I am asking:

Am I misunderstanding something and underestimating the potential of Called Shots, or are they indeed yet another feat-intensive, trap option present in Pathfinder?