In the past, for High Speed design (10gb+), I have used these materials at the engineers requests. There are no real issues with the Lead free process on these materials, and the Nelco 4000-13 seems to be widely used in these applications.
The fab houses did ask that if I did not really need Nelco 4000-13 SI and EPSI, not to use it - but not because of lead free issues. The problem with these materials is that they have a huge shrinkage issue in the x/y directions. This instability causes issues in lamination and layer alignment of features. It is very hard to work with, and often a test run of a design may be required to see if it will be manufacturable. For board sizes of about 3.0 x 3.0 you will not have issues, but for very large boards like the ones you do (18.0 x 18.0) it is a nightmare for the board houses to work with.
If you need the capabilities of the SI and EPSI materials, try looking into some of the Megtron materials. There are many users in the High Speed world today moving that direction.
Thank you very much for this info. Yes, some of pcb fabs complain that Nelcon 4000/4000-13 can't processed with good yield especially on the caseof pcb with buried via where need to do 2-3 times laminations. We are considering to adopt Megtron4 and Megtron6