Server script or client script is good for a full fledged development?

Hi one of our Clients Using ERPNext-14, They only use Client Script and Server Script for full-fledged development. I feel it’s not a good idea. what you feel about please drop your comment and suggestion please.

if it can be done via client and server script and for single site(database), if the code logic is not that complex, it is a viable customization option, I think.