What are the disadvantages of multi-tier application model?

Disadvantages: will have issues when the number of users gets big; has limitation to solve issues like security, scalability, fault tolerance and etc because it can be deployed in only 1 or 2 computers

http://www.codeproject.com/Articles/430014/N-Tier-Architecture-and-Tips#Advantages and Disadvantages of Different Tier Architectures

Well, there may be few.

Like we need to handle more coding.
Of course if we use more tier than required it will increase inefficiency.
And surely will create complex coding structure for the same.

This is only when we go for more tiers than required otherwise it will only give benefits. Its all about using logical mind. he he

Well, I think you got your answer, because I really have no knowledge about this particular query.