Do mental stimulants vary in effect?

Define precisely what you mean by “mental stimulants.” That could vary from “uppers” to a lecture by S Hawking.

Depends what you take, for sure. For example, you’re definitely more prone to health issues if you take amphetamines after being overly tired for too long. This is why you should always wait a few days between taking things.

Yes, otherwise, there would be no difference in value between caffeine, adderall, cocaine, and meth… In fact, cocaine and meth would likely cease to exist because why risk selling/buying something illegal if you could get the same effect from something legal and cheaper?