The most important challenge or if you want to call it a drawback of buying an existing website comes in the form of finding a suitable authority website in your niche. Although, you can get a lot of existing websites, buying an established website is a daunting task in itself. Then the second challenge or drawback is that it is really hard to get an undervalued site. You would not like to pay a high amount when you go out for buying a website. You will always like to keep your expenditure low.
Thirdly, you require doing extensive research before buying an existing website. You have to look into the history of the website, what is the website ranking, how is the SEO done, from what sources do it gets the traffic, how sound is the earning model, what is the technology used, what is the competition and things like that. When you have gotten answers to all of these questions, then probably you move closer to buying that site.
And last but not the least; you have to see whether the website content is original and authentic. You have to determine and make sure that the website you propose to buy has no copied content in it.