Well rock is quite common look at EdgeQuake A LOT of mons carry it. Ghost/Dark seems uncommon but ttar is used a lot with both edge crunch and pursuit which are quite common moves. Shadow Ball is also a move used a lot. Also fire type moves are careed on ALMOST EVERY dragon because of steel's resist.
So those weaknesses are INDEED common.
About Technician:
Silver Wind, Bugg Buzz is the same base power and has more pp so useless kinda.
Magical Leaf, Well ok this might be but Giga drain with another ability is more usefull.
Grass Knot, Not always 60 or less.
Ominous Wind, Shadow ball is only 10 base power less but more pp.
Shock Wave , Another ok but it has no real reasons to use it.
Aerial Ace, look at it's attack stat.