I am about to turn 60 years old... so I can talk about how things were done " in the old days"...

I am always worried when a rebuild is done without taking the engine block to be VATTED. This is a hot boiling out with everything which can be taken off the block taken off...
This was standard procedure in the old days... for every rebuild...
One area which is almost impossible to clean without vatting is the coolant cavity at the rear of the engine where flow slows down and lets junk collect... at some point affecting the cooling of the rear of the block and sometimes of the rear of the head depending on engine design.
Also, internal oil passages can need cleaning... often with a nice long wire brush... but I have seen times on old tractors where one needed to thread the available hole and use a grease gun to break out the obstruction...
Hate the idea of doing a great rebuild on all the rest of the stuff and still have unseen oiling or cooling gremlins lurking for some time in the future...