The idea that Mr. Spock would even lower himself to meld with such a Nirak (Vulcan for ‘fool’) is illogical. Nevertheless, if a meld was attempted, the resulting flood of illogic would be enough to cause Spock to go insane.
Yes, the mind meld can cause pain for the ‘melder’. Sometimes emotion can be passed from the ‘meldee’ to the ‘melder’. Since Vulcan’s suppress their emotions, this can be quite discomforting.
Star Trek The Original Series: Season 3, episode 8 - "The Empath"
On planet Mianra II, Kirk, Spock and McCoy find a mute woman who McCoy names Gem Gem's emotional system is so sensitive that it feels the pain of another and that pain becomes part of her, before she dissipates it. Because of this ability they call her an empath.
The Mianran sun is about to go nova. An advanced race called the Vians have the power to save only one species from the impending nova, and so they want to test whether Gem's species is worthy of being saved. The Vians want to be certain that she has the principles of self-sacrifice, the will to survive, the passion to know and the love of life. These qualities, they say, make a civilization worthy to survive. The Vians severely torture McCoy to the point of death to see what Gem will do. After brief hesitation, Gem "heals" Dr. McCoy.