Most indefensible thing is manufacturers did not create clean out ports in the manifolds.
50,000 mile durability testing made it undeniable that that gunk was going to build up.

This is like EGR systems 40 years ago,
manufacturers knew deposits would build up but did not create ports or non-threaded piping with quick disassemble clamping.

I guess you can even say it is like fuel injection 50 years ago
where cleaners like Techron were invented and put on the market only after customers had headaches for at least 5 years.

Give the military limited credit that they at least make an effort to make field stripping and cleaning routine - perhaps it is because it is a life and death matter.
Yes, even they replace reliable M14s with nightmare early M16s