There is also the drawback of having to obtain enough mentats for the entire game.

True.
Are they in limited supply in Fallout 1, or is there a slow but infinite supply?
Also, this wouldn't be exceptionally difficult if one's Barter or Gambling were very high.

In theory there is an infinite supply of all drugs, you just have to wait for shop inventories to respawn. Which is fine if you take drugs very sparingly, if at all, but try making a character who is dependent on them and there might as well be a finite supply for how often you will be in withdrawl.

It's possible I suppose, but convenience gets to be an issue (I think you even mentioned this in a character-build post somewhere; it's a good point). For IN you'd have to worry about buffing for each level up (to get your skill points? I can't remember if this works or not), as well as the dialog that requires the IN checks; then having to sleep it off or worry about getting addicted each time.

For CH a player might only have to do the trick two or three times, whereas for IN it might need to be done twenty to thirty times or more.