Frequently guidelines for ADHD will not mention methamphetamine at all.
Most likely most guidelines either exclude methamphetamine or place it as a last line treatment not because of evidence but because of the stigma around methamphetamine. There is nothing to indicate methamphetamine should not be a first line treatment, in fact it has fewer side effects compared to all other amphetamines but it can be expensive, especially in large doses (the normal maximum for ADHD is 20-25 mg/day, 15 mg/day for obesity, for narcolepsy several guidelines have indicated up to 60 mg/day may be required).
The methamphetamine made by the brand name manufacture (Lundbeck) and the generic (Mylan) are pharmaceutical grade. The process the companies make their methamphetamine is much differently that how illegal manufactures make it. Also typically illegal methamphetamine is made in the cheapest way, with the most easily accessible ingredients (which are often toxic.
So methamphetamine, like most drugs, can be made several ways and when it is made illegally it is made much differently than a drug company would make it. Also keep in mind that prescription methamphetamine is an oral medication, most illegal methamphetamine is smoked, inhaled or injected and in far greater amounts than would ever be prescribed. Smoking or injecting methamphetamine causes a massive rush, can result in dozens of health problems, and tends to result in an extreme crash. Oral methamphetamine (or other prescription amphetamines) does not give much of a rush or crash and therefore when used properly is far less addictive.
Res non per se at per usum, bona aut mala est.
Nothing of itself is good or evil; only the manner of its use makes it so.