One big problem; if you keep most of the Java code on the servlet side and you throw an exception, the stack trace will give you the line number of the offending code.

OTOH, if you do all that heavy lifting in the JSP side, depending on your choice of server, you get a stack trace...either without a line number, or with a line number of the JSP which was compiled and turned into a Java program on the fly.

And we all know what machine-generated Java looks like, right? Like a big plate of pain with some extra pain on top. Good luck bug-hunting that one.