Quoted from http://blogs.msdn.com:
User Instances (also known as RANU) available with SQL Server Express Edition, are NOT RECOMMENDED for hosting environments.
I have a server running Server 2008 R2 Web Edition with IIS 7.5 and Sql Server 2008 R2 Express installed.
I have only 3 friends as my hosting clients. Each has a restricted Local User Account on the server.
What is the drawback of using user instance in production environment?
Edit
Rather than considering the limitation in memory usage and database size that I know, let us consider the user instance only, why isn’t it recommended?
I think that the answer to your question comes with the definition of the RANU
What is RANU
and also read the http://msdn.microsoft.com/en-us/library/bb264564(SQL.90).aspx
The RANU starts a seperate instance Process of SQL Express for every user, so in a share enviroment with let say 30 sites that needs access to SQL, then SQL run 30 instances at the same time, that is takes as you understand a lot of recource/memory etc. Also as say in the article the first time SQL Express run for every user is make copies of all system database to create this user instance.
In a single user machine to run one process is normal, but if SQL Express starts to run many instances then you take a lot of resource, memory, and this is not easy to administrate, update etc.
In the article on the link there are more detail informations about this.