Only the ones you receive, IME…
And of those, there are -
Teeth.
Small mouths.
Poor technique.
Gag reflex.
Squeamishness.
“You will pull out before…?”
Impatience.
“My boyfriend/uncle/brother’s &c. is bigger/smaller.”
Actually attempting to literally blow… &c
To consider.
The good ones, though, are very good 