The issue is leakage and how to deal with tube expansion that occurs when the headers go through the heating and cooling cycles that they do everytime the engine is run.
Slip fit connectors were the easy solution for carbed engines as there were no sensors in the exhaust. Easy to install and service also.
Then came EFI with critical sensors in the exhaust downstream of the collector, in many cases.
The slip fit connectors leak. There is just no two ways about it. They may only leak enough to affect things when the engine is cold, as the inner ring expands more than the outer due to different temperatures once everything is up to temperature, but the fact is that they still leak at some point in the warming up process. The double slip reduces this, but it still happens.
The only way to 100% deal with it is to put something that is a closed, sealed pipe in the system that can also expand and contract. Hence the bellows comment.
These "bellows" pieces are getting cheaper as this is what is used, and has been for decades, in the imports like Honda and Toyota to deal with this issue. Take a look at a frontwheel drive car in particular, and it most likely will have a piece in the exhaust system that looks like a short section of very large AN stainless steel braided line. This is just a woven stainless steel protective covering for a bellows unit.
EFNFAST- per your question, it will end up failing because of the motion produced by the continual expansion and contraction and the nature of the "slip" fit. The slipping motion will eventually lead to the failure of the RTV. This is not the case on a bolted manifold or flange. In those cases there is little to no movement and the high temp RTV works well.
