Difficulties with Overcomes

I know some people have concerns that Overcomes are used for absolutely everything that's not an Attack, Boost, or Hinder.  Seeing it in play, I don't have an issue with it, but I wonder if anyone has used situational bonuses or penalties on them to reflect difficulty.

The book doesn't seem to speak a word about this and I've read the whole thing twice so I don't think I missed it (but I may have!).

I'm talking about a GM-imposed bonus or penalty that is not the result of some action in the scene previously (i.e., a Boost or Hinder).

For example, if the *player* did a kickass role-play while trying to persuade the mayor, should that merit a bonus on the Overcome?

If the hero is trying to disarm the most complicated doomsday device ever made, should that incur a penalty?

If Strong Guy who regularly lifts tanks suddenly has to lift a whole mountain, does that have the exact same chance of success? Or, alternately, if the GM says he automatically picks up tanks, but he has to Overcome to lift the mountain, at what point does he need to Overcome? What if it were "merely" a 1000-ton chunk of debris? And is it just as easy to pick up one as the other?

I know the answers to my own questions: "It's not really that type of game," and "You can do whatever you want in your game."  But I'm wondering if anyone has encountered these kinds of situations in your games and whether you just kept to the RAW or imposed some sort of bonuses or penalties to Overcome attempts.

I do a combination of the two, really. 

For the role play example, I'd reflect the results in the narrative. A twist could be that the mayor would love to help, but there's a third party or existing issue getting in the way.

For disarming a Doomsday Device, you can give challenges passive effects on the scene. For example, maybe if that hero takes a twist, it must be a major twist to reflect the added chance of things going horribly wrong.

For Strong Guy, does it make sense in the narrative for him to lift an entire mountain? Comic book levels of strength vary wildly even from issue to issue, so I'd just figure out what makes sense. It could make for a cool moment for him to lift an entire mountain, as long as it fits the narrative of the story. Alternately, you could decide that it just doesn't make sense for the current narrative.

Really, I play fast and loose with the rules when it comes to soft mechanics like this. There's just one important rule to Overcomes in my opinion:

Your success may be guranteed, but the consequences may vary.

A RAW way to make one task stand out as harder then others is to make it a challenge that has more then one box to overcome. For example lifitng a mountain might be overcome to start to heft, then overcome to keep your footing, then overcome to get the oompf for that last push.

Principles, meanwhile, are a way that certain tasks can be made easier cause they are in that characters wheelhouse. Superstrong guy has an easier time dealing with strength based challenges cause he gets to use his max die when overcoming with brute force, as an example.

I would personally have no issue with saying "hacking this computer is done at a -2 penalty because the security is that good" as another way to handle it

As a mild tangent, often times an overcome represents something that, in ideal conditions, the hero could do no problem but they are in the middle of a comic book scene. Maybe  super strong guy can bench press a car no problem. But now he has to do it with finesse so he doesn't squish the person under the car, or they are under fire from the laser robot stormtroopers, or he has to be careful  where he puts the car down so he doesn't squish fleeing civilians.