In short it was easy to misuse global v8::Persistent<T>`handles leading to memory corruptions and such. Unfortunately making them safe to use also means making them hard to use directly. Previously they were just interchangeable with normal v8::Local<T> handles for things like calls.
This is actually easy to work around by creating trampolines. Dan Carney creating a small header file with templates to create those trampolines easily:
That approach doesn't work (without a lot of effort) with templates at the level cvv8 uses them. There are literally thousands of InvocationCallback instances in the cvv8 tree, most of which are bound via the XTo templates, which run through 4 or 5 layers of metatemplate magic to do their things. It's a bit more complicated than a drop-in replacement can solve unless the drop-in replacement has the same signature AND type name as the old one (and even then it would require more effort than i'm willing to commit to).
