I've come to regard the electric guitar--my chosen instrument of expertise--as one of the lamest instruments around, especally from a compositional standpoint.
The acoustic guitar's big advantage is that it can play chords and yet be portable.
The piano allows for much richer contrapuntal performance than the guitar, but lacks that portability.
Most other instruments--flute, violin, saxophone (discounting the acoustic-guitar-like ones such as mandolin)--are monophonic (not violin, but close), but they are more expressive, especially because sustaining a note requires action, and varying that action can produce extra expressiveness, e.g. getting softer and louder over time; the instruments are "analog". The piano lacks this: one triggers a note with some volume/hardness, and one terminates it, and that's it; over time, the performance is discrete.
The gutiar offers a tiny bit more than the piano: it is possible to apply vibrato to notes in an analog sense over time; there are subtle things like sliding, hammer-on and pull-offs that allow transititioning between notes in different (but essentially discrete) ways; and it's possible to bend an individual note up two or three half-steps (but not down at all). Those exaggerated performance techniques tend to show up in monophonic lines more than chordal parts, and, except for note-bending, are not really all that significant compositionally. Perhaps Adrian Belew would disagree, but I can't play guitar like Adrian Belew.
I think it's no surprise that applying effects to the electric gutiar has been such a big deal. Arguably, it is the opposite--the electric guitar was the first instrument that needed amplification, and once it was being amplified, putting effects in the path was natural. But I dunno. Especially the ubiquitous volume pedal, which allows that analog expressiveness over volume.
Anyway, what got me onto this was something that came up a while back, which is that I can improvise better monophonic melodic parts on the keyboard than I can on the guitar, despite years and years of practice on the guitar and very little on the keyboard. Correction: assuming the improvisation is in C major or A minor. (I transpose the keyboard as necessary.) I still use accidentals; it's just that's the only layout where I know by heart way they are.
I know where they are better than I do on the guitar. On the guitar, I have to (and have) learn a complex pattern of notes to account for the way octaves don't naturally repeat up the guitar. (They do up an individual string, but that's only an octave and a half long.) So I know three different positions of major scales pretty well, and I know offhand where the flat 6th is in one octave of one of those three positions. Most of the time, I have to stop and think "ok, right, flat sixth is up a half step from the fifth". Although mostly in my mind I'm not thinking "fifth" and "flat sixth", I'm thinking "this sound" and "that sound".
So it just bugs me that the guitar doesn't really have any advantages--an electric guitar and amplifier are still more portable than a piano, admittedly, but not compared to an electric keyboard--and yet is so much more complex to understand simple things.
Admittedly, I can bang out arbitrary chords faster on the guitar, but once it comes to adding in some strange accidental to the chord, you have to stop and figure out a fingering that can make it work--usually much easier on a keyboard. And banging out arbitrary chords just isn't all that interesting, even if it is what a lot of guitar bands tend to do. (The good ones don't; compare, oh, Sonic Youth or Come.)