MLBPA chief Tony Clark spoke on a number of issues, and seems to come down against the change to the pitch clock:

“That’s a conversation that should have warranted a much longer dialogue than what we had,” Clark said Saturday. “We voiced those concerns, players voiced those concerns, and yet, the push through of the change to the pitch clock still happened.”

…

Clark’s main concern is that pitchers have less time between pitches to recover, particularly when maximum effort and pitch velocity are so important.

“When fatigue happens, you’re more susceptible to injury,” Clark said. “We’re seeing a lot of injuries and we’re seeing them in a way that simply can’t remove the question of whether or not shortening recovery time is in anyone’s best interest.”

Chron.com

He is also very concerned that the Athletics situation regarding a location for 2025 is not resolved.