Differential skills & quality depending on medium & subject.

One of my recent post on goodart discussion group

I find that I have a vast difference between the quality of art produced
depending on the medium I use and the subjects that I draw. Has anybody
observed this ? Is this common among artists ?
1. I am good at portraits & figures, but bad at landscapes
2. I am good with pencils & pastels, but bad with watercolors
3. I am good drawing complex surfaces of a single object
(portraits/figures), but am not so good at
drawing a composition of simple objects (eg. a sketch of a living room)
4. I find myself to be better at drawing figures in watercolor than
drawing landscapes with the same medium.
5. I find that drawing comes very naturally to me without any training
(except for reading a lot of books), but I
can't paint well in watercolors in spite of a lot of training.

I have following questions.
1. Have you observed this quite often among fellow artists, yourselves
or your students ?
2. I want to start doing portraits/figures in oil, but I am in mixed
minds. Are oils more close to pastels, or watercolor in terms of
execution ? On one hand, I am afraid I won't do well as I haven't been
good with watercolors, but on the other hand I feel, it might be ok,
since it is possible to do lot of corrections and layers with oil.

Note that I am an amateur, doing portraits as a hobby. So probably I
haven't put enough time in the other mediums as required.