[Paul M]

Originally Posted by vijayninel

[*]Complete new software written from ground up. As mentioned by Wayne Luke

Its not completely new, its being re-written in stages.
The system is driven by templates, which in turn call the API and manipulate and present the data. Therefore the template system has been vastly extended, and the API is new, but there is still an amount of legacy code in use (more vanishes each week). This is especially true in the ACP.

Originally Posted by vijayninel

[*]As it is a completely new software, it will be incompatible with all vb3x and vb4x mods.

Yes, the old php hooks are all gone, so anything that relied of them will not work.
Also the old template hooks are all gone (there is a new unrelated template hook system).

In theory, its possible that a very simple old modification would still be valid, but very unlikely.