Amphetamines, I think, due to the amplification of one's senses and perceptions, could, in theory, make one unhappy on account of a stronger perception of a reality you might not care for. Perhaps, too, the energy and stimulation induced by amphetamines, could create a state of unease, perhaps because one has no appropriate channel to use that extra energy and stimulation.
For me, amphetamines, at higher doses with respect to my tolerance, do induce a definite euphoria every time, however, there have been times where, even though the amphetamine should still be quite active and I am not really coming down, I do experience a bit of dysphoria and unpleasant sensations, usually ones of fear, nervousness, paranoia, and a certain blankness. It is not usual, but it has happened, and in such circumstances I really just want to come down. But then, the comedown is most certainly a depressing experience, for me, laden with fatigue and pseudo-narcolepsy and psychomotor retardation. This can last for a few days, but it tapers off in severity fairly quickly.
With methylphenidate, I never really had a debilitating comedown, because it came on so quick, and it resolved itself by the time my nighttime cocktail of sedatives would wear off; with methylphenidate, there is a quick high, a lot of physical euphoria, and then within an hour or so, for me, when insufflated, I began to feel the plateau starting to drop - within two or three hours, with comfort meds, it was as if nothing had fazed me.