Dynamic extensions don't work properly and produces a lots of crashes which compromise the usability of ROOT 6.
A very simple reproducible is to type (or add in a macro)
auto h1 = new TH1D("h1","h1",nbins,0,1);
which crashes in ROOT,if you do before
root> .dynamicExtensions 0
you get a clean error message
ROOT_prompt_1:1:30: error: use of undeclared identifier 'nbins'
auto h1 = new TH1D("h1","h1",nbins,0,1)
We could discuss the support of them in un-named macro for some backward compatibility issues, but in named macro we don't need this feature.
I see this as un urgent issue in order to have ROOT 6 usable for analysis.