Metal Gear can be a tough game if you didn't get the instruction book or map with the game. People who borrowed, rented, or bought used found it hard.
I found it fun though and unfortunately it becomes "easy" when you figure out everything. Unlike other Metal Gear, the original one had fixed enemy path and fixed pattern plus fixed rule set so you just needed to know where to go, what to do, what frequency to use, etc and it's doable to finish it without taking potty break.
If this was ported to TG-16, it most definitly would have better graphics. I remembered how it looked "grainy" like the programmer was trying to push the limit of NES. Back when the game was made, bankswitching was done with a bunch of logic chips, before MMC chips was developed to be used. If Metal Gear was reprogrammed for MMC1 or later, it may have looked better, less grainy, and a lot less wait in between scenes (that one second black screen was like trying to play off CD game)