First off the difference between babbitt and inserts.
Well properly done you should expect between 50,000 and 80,000 or so miles on the bearings. Of course the catch phrase is 'properly done'.
Babbitt has some special advantages. The engine is designed to eat dirt, it was designed to run on dirty low quality fuel on dirty roads with low quality oil. That was 1928. Of course you must understand the engine was designed to run 60 MPH getting better than 20 MPG.
So the babbitt allows for embedding. This lets large chunks of stuff to safely embed in the thick babbitt and not harm the crank. Babbitt also fails gracefully. It lets you know when something is bad and will likely let you limp home. Ya, it can fail suddenly, but it is likely you missed the starts of something many miles earlier. When it fails the parts are not generally ruined.
Babbitt is happy with the oil system. No pressure needed the bearings are well bathed in the slippery stuff.
Inserts are done cause they are modern and too many guys have done babbitt wrong. Inserts done right are fine- but. The inserts do not allow dirt to embed. They do not fail gracefully, if they are going to fail it is likely catastrophic and it is also likely to take out the block and the crank. You will need a tow home.
So whats up with babbbit. Lots of bad information via word of mouth. There is a lot of rules that must be followed to get a good pour. It gets even trickier when you are working with small pots of babbitt. Your goal is to pour the babbitt in such a manner that the crystals are small.
So this means you need to have very clean pots, small chunks of babbitt so they melt quickly, a hot burner, proper stirring techniques. If you are running a small pot you have to be even more careful.
We did our first pouring long before the internet. Lots of things went wrong. We now understand why since we have located technical info on the net. Finding the info and cross checking it was not easy.
Then there are the other details. Like proper peening tools and timing. Must be done while the babbitt is still warm or you cause micro cracking.
There is a lot to proper babbitting.
Then there is the thousands of dollars of tooling and knowing about how to use the tooling. The knowledge of proper machining techniques is also a killer. You have to have a deep understanding of what the tooling is going to do and how to compensate for the induced errors. There is a saying brand new machines in un-experience hands will not be as accurate as worn out tooling in experienced hands.
Just getting the bearing inline and the proper position is just one small piece of the puzzle. There is understanding shims. Knowing about caps that are too messed up to use. Realizing that Ford used what consider racing tolerances in building the A engine on the line. You have to check the crank grinders work as it needs to be to thousands or sub thousands accuracy to return to factory specs. In some cases you are lucky to get 10's of thou accuracy. Then there is balancing. The piston rod combo on the factory line would have been within 4 grams of each other. Today you are lucky to get the rods in 5 grams total weight (factory was +- 1 gram at each end).
Least I forget one other factor. If the babbitt is lead based do not bother using it unless it is a museum piece that is not driven.
I hope that brings some perspective to what it takes to build the A engine.
It is not easy or cheap to do it right. It also happens that the A is such that it will 'work' when it is way out of spec. So it can be thrown together and made to move under it own power.