I am no CFMEU fan, far from it, but lets put this into context.
20% over 4 years is about 4.7% a year compounded. As an ambit claim that is pretty standard for what we are seeing in all the Union claims now given the inflation rate. The Nurses union, Police, ambos are all asking for more when you factor in all the other stuff included. The nurses union EBA that they just voted down was more than that.
There are of course arguments about worth of each of those but the claim is not an unusually excessive one as an initial ambit claim.
Dan Andrews was initially supported by 3 major unions. The ANMF (nurses), the Firies union and the CFMEU. He delivered all the changes related to the CFA to the Firies and Lisa Fitzpatrick of the ANMF was colloquially known as the deputy premier behind closed doors when he was Premier. The new Premier has no such loyalties, their power is diminished.
The difference is also that the firies and nurses are public sector unions mainly so the government has a say in their EBAs but they don't in the construction industry.