It is very hard to list all features not supported for VT300's, because there is no complete list of them.
Basically, advanced screen features (so called ReGIS) are not supported. You can find more info about ReGIS at http://www.vt100.net/docs/vt3xx-gp/chapter1.html
However, VT3xx inherits main functionality from VT2xx and older. From the following feature list almost everything is supported, see http://www.vt100.net/docs/vt320-uu/appendixe.html
From the above, these are not supported currently:
1. not all NRC sets
2. DECSCL (Selecting an Operating Level)
3. DECDLD (Down-Line-Loading a Soft Character Set)
4. DECSASD and DECSSDT (Status line display)
5. DECDWL and DECDHL (Double width/height characters)
6. Selectively Erasing Text
8. VT320 Reports
Simply, if your host program requires VT3xx, the Rebex Terminal will probably not work. If your host program can work with VT2xx, the Rebex Terminal will probably work without any problems.