A major challenge facing today's Sn/Pb assemblers is the limited availability of Sn/Pb components. Why is this an issue? The first is the concern of Sn whiskers when components are plated with pure Sn (the popular alternative to Sn/Pb). This is a potential reliability issue specifically for assemblies that are required to last many years. The second problem is with BGA components. For Pb free, they are typically bumped with Sn/Ag/Cu solders. These solders melt at 217 C which is typically higher than the peak reflow temperature of Sn/Pb. If your BGA supplier no longer sells the Sn/Pb version, you could be forced into a mixed alloy system which is probably less reliable than all Sn/Pb or all Pb free.
The Sn whisker challenge can be addressed with a number of mitigation techniques, but if you are building something with extremely high reliability requirements, you are better off staying Sn/Pb. From the BGA perspective, you are probably better off going to Pb free to avoid the mixed alloy system.
I remember talking to companies that were ecstatic that they were building something that was exempt from RoHS legislation. Unfortunately, today they face a new set of challenges that weren't there a couple of years ago.