uncommon sense

This.
The monsters everywhere.
How does your world/kingdom/country even function like this?? Why haven't soldiers come through and wiped everything out if it's such a problem for commerce? Why depend on adventurers and not army? What are you using those taxes for King?!

Though lots of authors try to explain this away, alot of other authors don't and just expect the reader to accept it, which we do because we're used to it.
Thus, it's uncommon sense.
When you say high end armor, what kind do you mean? 

oh... according to wortenia senki, this has happened in the past with liu bang being sent to bashu, but i am not sure what that was about...

the thing about raising the rank though (in WS), is to have them "officially" become part of your country
before this happened, the MC could not be knighted, could not leave the country, and cannot be killed
being knighted is impossible since... i assume this is because the MC is stronger than a knight
the MC cannot leave the country because... i assume that this was done to prevent hostile countries from hiring the MC
the MC cannot be killed since he is too strong, and even if they can kill him, it will only cause a reason to doubt your country

giving the MC an isolated territory is basically "politely" asking the MC to stay there and not interfere with your own territory 

Usually that's because of a reason like "we cannot afford lose you", but the DLs as of late aren't like this anymore, right?
Moon-led Journey [...] (Tsuki ga Michibiku [...]) is a good example of this, though many other stories seem to still have these uncommon sense. 

I might be missing something here, but the only time they explicitly say what type of armor was used there is against mail, which mild steel arrows could penetrate. Otherwise thanks to missing information (and that historical sources are very often inaccurate, lies or leave out important details) there are to many questions in the story. Thin plate could be penetrated more easily then thick plate, but the quality and strength of the materials materd allot. for example antler is supposed to be harder then mild steel, but not as hard as hardened steel.

Though I just do not trust any record without testing to show what was going on. I could easily see people going against "ignorant savages" to buy cheaper armor thinking they did not need the extra expense, or thinner armor/padding that would cause less overheating, etc.

But ya, if someone develops a weapon that their armors can not adapt to, they will drop the armor since is encumbering over time (and has other issues like heat regulation) 

Here are some that bugged me for a while
1. Cultivator are immune to bullet
2. Monster that come from another world are immune to modern weapon (some novel even said nuke doesn't kill it)
3. Walking through vertical wall without rope just pure agility 

Almost instantaneous raising of rank to a noble despite having no history in the kingdom they are transported into. Language is the same across the entire world with no variation. Deadly monsters everywhere yet farmers and ranchers are only ever bothered by seemingly weak ones if any at all. 

That there can be a man-made disaster the destroys a massive magic civilization in its entirety, but yet, somehow, humans survived through it. (see Shikkaku Mon no Saikyou Kenja~ Sekai Saikyou no Kenja ga Sarani Tsuyoku Naru Tameni Tenseishimashita~) 

Pretty dumb that cultivators spend hours flying on their stupid swords in the sky and can still remain white as snow lmao, my ass would be sun burnt like a lobster in 20 minutes