Caching is the best available tool to enhance the performance of the app. ASP.NET is very popular among the developers community because of its benefits. But along with its some of the rich features, asp.net cache has a few limitations as well. some of its limitations may include,

1-ASP.NET cache has Data Integrity Problem in Web Gardens /Web farms.
2- Cache size limitations
3- Single point of failure
4- Scalability problems

One of the main reason for these limitation is its stand-alone and in-process nature. so asp.net is God?s send for smaller web farms. But when the number of servers increases from a certain level, scalability and reliability issues may pop up. In such conditions, the use of a third party distributed caching solution, like NCache, can be a better option. Below is the link which ll explain my point further.

http://www.alachisoft.com/ncache/asp-net-performance-scalability.html