The most deadly thing Wily can muster is death traps in the floor, and Yoshi's gaming debut was a kick to the crotch for every villain who relies on those.
Love the lil guy, but Yoshi can still, among other things, get crushed, die in a pit of magma/lava, plummet to his doom in a bottomless pit, or get eaten by something larger than him (like those bastard fish in Yoshi's Story).
If Wily got together anything similar to those, or better, Yoshi would be in a pickle...
axing his claustrophobia (necessary to enter Wily's fortress)
I always did wonder about this in SMW.
Yeah, Yoshi, I can take you underground, or in underground pools of water, either of which can be quite cramped. I can also freely take you into the Sunken Airship, which is rife with ghosts.
So, really...WTF is up with you, Yoshi?