A long history of not working particularly well. And testosterone hacking is the grand-daddy of them all. You know all those steampunk-y horror stories about rich Victorian eccentrics who go mad because they implanted monkey testicles into themselves in a bid to achieve eternal youth? People actually did that.
There's a thin line... or rather let's say a fork in the road between pseudoscience and science. I think they both start in the same, non-scientific place. And that place is a kind of magical thinking. It's what happens after the initial inspiration that makes the difference between science and pseudoscience.
Testosterone is higher in men than women, an in particular higher in young men, and higher in men who achieve social dominance. It also rises after sexual intercourse. So by magical thinking testosterone must be the "manliness" hormone; it makes you young, vigorous, dominant and sexually potent.
There's nothing wrong with that as a starting point, but in the long history of testosterone hacking it hasn't worked out, except for gaining muscle mass in conjunction with resistance training. Sure if you treat men with a testosterone blocker they'll eventually lose interest in sex, men generally have less sex as they get older, and at the same time their testosterone levels decrease. So it's natural to jump to the conclusion of a chain of causality: worn out old glands put out less testosterone, and that causes a reduction in interest in sex. But if you actually test that hypothesis, it doesn't work out: individuals with least decline in testosterone levels actually have less sex than their normal counterparts. The normal decline isn't large enough to produce on its own any measurable effect in interest in sex. So if your interest in sex is dropping as you get older, look elsewhere for the cause.
This is the danger of calling testosterone "the male sex hormone", as if God had a punch list of features He wanted and implemented each feature with a single steroid compound that works in isolation from everything else. Yes, testosterone is involved in masculinization of adolescents, but it's not as simple as the more testosterone you are, the more male you are. The endocrine system is complex and dynamic, responding to internal and external changes -- including aging. Both men and women have and need testosterone and estrogen, in ways we don't fully understand yet.
If you want to get the most out of your life, eat a variety of real food in moderation, exercise, sleep, and in general alternate stress with rest. The ancient Greeks could have told you that, and in thousands of years we have not been able yet to improve on that as far as lifestyle advice is concerned. Go to your doctor if you're sick or injured, or even for a testosterone treatement if you have abnormally low testosterone for your age. But don't go to the doctor for a magic pill that will make you youthful, manly, sexy and dynamic. All that's up to you.