You can't find problems unless you pull the breeches or you discover the fouling trap the hard way.
Enclosed are some pics I took of a very big name high end priced mass produced ML breech from a rifle that came into the shop for inspection in 2003. Two as removed and one after cleanup.
Its poster child for the typical, poorly designed and installed mass produced percussion breech.
Note the cross drill, poor ignition is the usual result especially if a number of shots are fired, though this may be relative. Note the large fouling/rust trap. This traps fouling and also cleaning solvent and oil which makes for increased corrosion and more misfires from contaminated powder charges unless great pains are not taken to get the oil out of the "trap" before loading. Note the fouling encroachment on the threads, this is relativly minor compared to some I have seen.
Fortunately the rifle was not used with some of the "replica BPs". AS it is I am surprised the corrosion is not more advanced but water based claeaning followed by a decent gun oil on a dried surface will prevent BP fouling from doing much dmage even if its not all removed.
I would also point out that sloppy assembly such as this also increases breech thrust on the plug since the gasses had a larger area to push against.
Yes I been shooting MLS since about 1964. I have been taking them apart and putting them together since about 1967. So rather than tell me how good your luck has been, pull a breech or two and tell us what you find. Maybe you'll get lucky. But most will be found to be similar to this unless I am mistaken, which I doubt.
It is simply impossible to make a mass produced ML and sell it for less than the parts cost to make a high quality rifle and not cut some corners.
Took me a while to find the pics so forgive my delay in responding.
Dan