Long Keyword

Long tail keywords are defined as keyword phrases that consist of between two and five words, usually used when searching for a rather specific item.

Long tail keywords are useful, before start work directly on generic keyword or main keywords it is beneficial to start work on long tail keyword which include main keyword in it, it can easily get high rank on search engines

It depends on your nature of business however long tail keywords always easy to rank and i think you should rank first of all long tails keywords then try to rank short keywords but it should be relevant to your business and website.

the biggest disadvantage,this limit the search result of the keywords and this may effect the ranking.

Long tail keywords are Google because law competition but they have law volume too. You not have to work hard for long keywords.

I use a long keyword for my website?

Long Tail Keyword has better conversion rate.

I think using long tail keywords is beneficial for website. It is the combination of many works and thus have chances more than single word keywords to be show for a query.

Having some long tail keywords may make users to land on your page directly from SERP.