Approval or criticism has depended entirely on a given government's performance.
Not quite. Solidarność went very left wing (far more than the other unions), and thus they will criticise anyone that doesn't share their hard left wing feelings. But it's a recent trend, as they've previously been quite cooperative.
Solidarność was part of the government of Jerzy Buzek
As I recall, that was an example of Polish politics being downright weird, as a trade union had lurched heavily to the right.
and genrally supported the PiS govenrment.
Wasn't there some strikes involving Solidarność during the PiS government? From what I remember at the time, Solidarność were much more to the left while PiS more or less occupied the same centre-right ground as PO. I definitely remember that Kaczyński refused to negotiate with different groups.
It has been critical of the anti-worker policies of the Platfusy.
It's only in recent times under Piotr Duda that they've gone very much back to the militant left wing. Krzaklewski for instance was very centrist, I think.
What trade union would support pro-oligarch, pro-corporate and anti-worker policies?
It is true. In a sense, Solidarność remind me of the UK RMT - there was a guy called Bob Crow who was very similar to Piotr Duda's militantism.