Dear Nana,
it's an interesting topic. Let us proceed.

I think you are using way too specific of examples though... a scale is very different than a scale etude. The Chopin Op 10 #8 combines scale and arpeggio technique, and the Villa Lobos, by the looks of the score, has scales among many other techniques. The scale parts (E major at the beginning) wouldn't be out of the ordinary... most of the second and third page are broken or larger chords. The two are too varied to compare 'difficulty'.
I'm comparing the scalar passages in both etudes, and both have some mixing with other stuff. Why I did choose these example? Because they present common usage of scales in actual music. Although there are "pure" scales in passages of the repertory, they are less common.
Now, about the E major scale at the beggining, and the other ones, they are anything but ordinary. It may looks quite nothing on the score, but on the guitar it is a huge nightmare. You have - as in the Chopin - to begin full throttle,
forte,
legato (which is a beastly problem on the guitar), and in a demanding speed. Then you must repeat the idea musically (the transposed scale), but the way you produce it on the guitar changes (specifically, there is no way to repeat the exact relation between the two hands. So, you have two different passages that must sound the same. To add a twist, you have some ornamentation, leaps, chords...
I'm talking about regular scales. I don't know how a major scale suddenly becomes a 'pop' scale, when it is the same major scale that comes up in classical music. What about an octatonic scale or a whole tone scale? The T-S-T-S-T-S pattern going up can be used starting on any fret, and going from there (without using an open string)... obviously the right hand has to pluck/pick or whatever but the actual pattern itself is visually easier to retain than on the piano (for me at least).
If you give a piano student with one years experience the C blues scale, then ask him to play it on F# just as quickly without mistakes, then give a guitarist the same task, I don't see how the pianist would have an easier time.
Well, it is a matter of opinion of course, and I respect yours for sure. But I think we reached the point I made earlier. A major scale is a major scale, but usually - in pop music - guitar players use patterned scales that are always the same. In this sense, I agree with you completely: a patterned scale plucked on the guitar is quite easy, because it never changes. Anyway, you can't compare this practice to scales and scalar passages on piano classical repertory.
The other way around would be: piano scales are so easy, because you have the major scale on the white keys, and the pentatonic on the black keys, and it never changes.
Although these patterned scales are...er...patterned, they are completely useless in concert repertory. First, you never study all string combinations that a 2 or 3-octave scale may have on the guitar. Then, the left hand fingering is a set of very little streams, and finally - and the single most complex problem in guitar technique - the right hand fingering is a complete mess. In a demanding scale (occurs me, besides the Villa-Lobos, the ones on Rodrigo's
Concierto de Aranjuez, or Turina's
Sonata, or even Berkeley's
Sonatina), you use every eight available fingers in many combinations.
Back to the piano, please don't think I disregard scales as a problem. My point remains the same: scales on the guitar are the most difficult topic of technique, while on the piano they are not.
The blues example is a perfect one: the blues scales on the guitar, specially with a plectrum, are piece of cake. You are right. But where does you use them in classical repertory? But we must dig deeper. The transposition you mentioned, from C to F#, would probably lead the guitarist to another pattern. So, if s/he doesn't know this new pattern he is as clueless as our pianist who doesn't know the F# scale.
It leads me to another consideration: when you solve an octave of a scale on the piano, you solved everything. On the guitar, every octave is a complete different story, and to play a 3-octave scale (A major, for instance) involves learning and memorizing a stream of 22 double movements. You won't find anything scale related on the piano.
Nevertheless, it's my own experience, and I'll rest my case before (I hope

) I start bother somebody.
Best regards,
Jay.