Also, if you need accuracy stay away from questionable vendors of 3rd party products, as much as possible since they are chaos generators on any project involved.

In my work we have our core banking system designed in 80s on top of Oracle DB so everything is just boxes around it, with corresponding flexibility towards modern development methodologies. The complexity of just doing a trimmed copy of production servers for say user acceptance test phase is quite something, connecting and syncing to hundreds of internal systems.

Needless to say estimates vs reality have been swinging wildly in all directions since forever. The processes, red tape, regulations and politics are consistently extreme so from software dev perspective its a very lengthy process while actual code changes take absolutely tiny time in whole project.