I have had 2 hair transplants. The first was shit because I went local and cheap and was stupid and the 2nd was to improve the situation using a reputable surgeon in Turkey. I have also had something called SMP done which is essentially a hair tattoo that most people get to create the illusion of a short buzzcut but in my case it was to increase the appearance of density of my hair which is quite a new thing. Examples:-
Shaved head SMP
Thickening SMP
The problem with hair transplants is they only really work if you have a considerable amount of hair left which is what I had because each person only has a limited amount of hair in the donor area which is the area at the back and sides of the head that doesn't go bald. For example if a guy was fully bald on top or about to become fully bald very soon the best he can hope for via transplants is probably a thick high hairline which in most cases unless the guy has a great face would look even worse than having a fully shaved head in which case a good hair system (wig) would be the best option.
There is also now however the option to use body hair, leg hair and beard hair which would be a great option for hair anywhere besides the hairline because that needs to look more realistic. And if you combine this with SMP which can also make a depleted donor area look thicker you could technically have a relatively full head of hair no matter what the situation.
The problem is money in a lot of cases. Surgeons in the US an UK charge diabolical prices for mediocre results compared to the better cheaper results you get from the much better surgeons in Turkey. Either way for all of the above you will be spending 10k at least if the situation is bad.