Indoor Plumbing
Believe it or not, there aint no indoor plumbing and their aint no double-ply-bog-roll.
You dont get the comfort of a BMW or Landy or a pair of Adidas/Nike.
No golf courses, no Nintendo, no radio.
If you could take a windup radio... you've still got no powersource.
Are you truely a mechanical engineer to duplicate 'engines'?
Can you actually build an aeroplane?
Have you the machines to build standard grade nut/screw/bolts ?
Believe it or not, modern civilization depends on something as small as the nut, screw and bolt. Thousands of people labour every day to make seemingly the most trivial thing, yet without it, society is at a serious loss.
Refined copper wire? Magnetic inductors? Boiler making?
Baah, 1 man going through a portal is not going to make any of those in his lifetime unless he dedicated his life to said work.
As a society we stand on the heads of our parents and ancestors. They built an economy that we can take forward.
Moving to some 'world' of darkness, or some D&D verse, would be a shocking loss after the first year.
You might be able to win the technical backing of one king, maybe two, but the conflicts would put an end to their society.
You promote democracy? Treason!
-Tim