HT can work well for guys with specific characteristics. The problem is that HT are marketed to everybody, whether they are ideal candidates or not.

Some things that make you an ideal candidate IMO:
Isolated frontal hair loss, not a very high Norwood
Plenty of hair remaining in the crown and parietal humps
Not a diffuse thinner
Above average donor density
Coarse caliber hair
Rate of hair loss is extremely slow or halted

Other factors:
Wealthy enough to afford multiple surgeries
Not in a hurry or desperate (which makes you susceptible to fraud)
Not trying to regain a teenage hairline
Not a perfectionist when it comes to your appearance (HT are not perfect)
Won't be bothered if some people know it's a transplant
Can wear a hat at work during healing phase/first year if needed.
Can take time off from job for surgery and healing

IMO the best results are seen with patients who have ideal characteristics; The fewer ideal characteristics you have, the less ideal your transplant results will probably be. IMO the "average" guy will get average results, which aren't worth the risks. (When you see patient photos of good results, notice that they are usually people with ideal characteristics to begin with.)

Some of the risks are:
Unnatural-looking results (the #1 risk, IMO)
Shock fallout of existing hair
Bad scarring
Poor graft yield