I wouldn't mix Scala and Java in a project. Calling Scala code from Java is complex and way harder than the reverse. You'll probably waste your time to glue together the code. Although I think lift will get most of the work that you need done it's surely not as established as Java EE. It's a server side technology with heavy use of templates. Security and scalability are very good. As I've only played around with it out of curiosity I can't tell about resource management and transactions. Reliability is up to your code and your hosting provider.
About Scala + lift in general:
Using Scala after Java is like breathing fresh air. To speak a little heresy: In my opinion Scala + lift is easily as beautiful as Ruby + rails. You have native xml and therefore html support. Scala is the most complex but also the most elegant language I've ever written in and lift makes full use of Scala's potential.
Update:
here's an interesting quote for you "Lift applications are packaged as WAR files and can be deployed on any Servlet 2.4 engine (e.g., Tomcat 5.5.xx, Jetty 6.0, etc.) " it comes from here http://demo.liftweb.net/. Writing the code in Scala but using Java EE services is possible but at least in my opinion not a good choice. If you want to use Scala you should stick with its own technologies. Scala is far superior to Java and has syntax features Java frameworks can only dream of. Lift makes use of them. Moreover as you're meantoning transactions I guess your project will be commercial and at least medium sized. That makes Scala + Java EE a difficult choice. Many professional Java developers will have problems reading your code and therefore getting help will be troublesome.
