I've ordered a M&P 340 which came from the first production run. I've used it hard. Never had a problem with the ILS.
You really have high hopes to "settle this debate once and for all"? Really?
Having been through a S&W revolver armorer class, I had a chance to exchange info with other LE armorers. I've also spoken to different folks at the factory this subject (being curious), including armorer instructors and repair techs. I've also seen a lot of new-style revolvers with the ILS come through range sessions. (None have acted up for their owners, yet.)
A potential problem with taking a non-scientific "poll" of private owners is that you have no way to knowing the actual circumstances involved in any reported instances of ILS "problems", nor the ability of any particular owner to properly & correctly identify a range of problems which they might assume are "lock problems".
I've learned of new-style revolvers being returned for warranty repair for owner-reported "lock problems", but when the guns were examined at the factory it turned out that it wasn't a lock problem at all.
I remember when the owner of a new 617 brought his revolver to us (another S&W revolver and I) because it "wasn't working right". Looking at the gun, the cylinder wouldn't carry up, and the trigger and hammer felt as if they were catching on something, and then quickly they wouldn't move at all. Opening the sideplate, it turned out that the trigger hand spring had become bent, mangled and then "cut", and had eventually jammed the mechanism.
That must have been the time for spring breakage, though, as it was about the same time that another guy brought in his mid-70's vintage M36 nickel because the cylinder wouldn't turn unless you tipped the gun at a downward angle. (Hint) The old-style hand spring had snapped. The broken piece was glued inside the frame by a few decade's worth of goo.
I had an old-style 649 that had the trigger & hammer lock up tight. It was a pre-MIM/ILS model I'd bought new. The revolver armorer of that time easily corrected a couple small problems.
FWIW, I've personally experienced, and witnessed as an armorer & firearms instructor, a LOT more problems with the older style S&W revolvers (60's-90's) than I have yet to see with new-style MIM revolvers. A number of things can occur that can cause a revolver to "lock up", even if it doesn't have a lock.
FWIW, the itty bitty dog-leg spring that anchors the locking arm ("flag", if you'd rather) slips under a recessed spot in the channel in which the bolt moves. I noticed during the armorer class that if you opened the frame, removed the hammer and bolt (as if some owners don't do this to "polish & improve" things
), and then tipped the frame on the right side, the locking arm could shift (of its own weight) and pull the spring leg out from under the recess (no bolt next to it). If the locking arm isn't properly installed (anchoring the torque lock spring's bottom leg), the locking arm has no tension holding it down under recoil forces.
People are going to believe what they want to believe, or what they feel comfortable believing, or what makes them feel good believing ...
That original M&P 340 I mentioned? I later bought another one when the No-Lock model was released. Thought the No-Lock variation was sufficient excuse to buy another one.
The one I shoot the most and carry the most? The one with the ILS (lock). Go figure.