It is a very loaded question and can end up with a never ending answer because there are numerous reasons why a hair transplant can work or, cannot work.
A hair transplant relies on genetically strong hair being moved and placed in an area of hair loss, where it will, after a period of resting begin to regrow and continue to fall out and regrow as if it were in its original area of the scalp. That is a very basic why hair can be moved and will regrow, because some of our hair is not susceptible to the male pattern baldness gene the hair can be used.
A hair transplant is a relatively minor surgical procedure, but carries many areas within the procedure that can damage the grafts and cause poor growth, and a poor result, so in that case you could say it didn ´t work. Graft handling from extraction, to cleaning and dissecting, to how they are kept whilst out of the body to the placement back into the new area can all create potential problems, and at any time the grafts should be prepared and handled under magnification.
Post op care is vital also, especially in the first few weeks after your procedure; if the grafts are damaged while they are healing could reduce the yield and reduce the quality of your result. So, the doctor/clinic and the patient both carry a responsibility in achieving a successful result.
Are you a good hair transplant candidate? Aspects such as age, hair characteristics, donor hair density, and present hair loss stage, potential for future hair loss, reason for hair loss, especially if not male pattern baldness, and very importantly your goals.
Technically you may be a good hair transplant candidate but if your goals cannot be met then you become a bad candidate. Almost in every case there will be a compromise in what someone would like and what can realistically be achieved.
As it ´s impossible to replace like for like hair, a hair transplant relies greatly on the illusion of density, as long as enough density is placed to block the light from reflecting off the scalp an illusion of hair density is achieved. Coverage over the scalp cannot always be even, and for many a lower density has to be placed over the crown, especially in advanced hair loss stages.
This just touches the surface on what could be a book to go into the question comprehensively. Not everyone is a good hair transplant candidate, so does a hair transplant work, on many levels no it would not work for some; but on the right candidate it can give a successful result. One of the keys is researching, and understanding as much as you can, don ´t rush and don ´t make your decision based emotionally, but what is possible.