I am very comfortable in using Synchro for analyzing signalized networks. I find it to be very reliable for that purpose. I am not familiar with the methodology Synchro utilizes to evaluate roundabout operations, so I am less likely to use it for that purpose.
From what I understand, each of the roundabout analysis tools available has its own strengths and weaknesses. The supporters of the different methodologies can be very passionate.
Any design process is somewhat iterative. I might accept the Synchro and SimTraffic analysis as a preliminary check on the feasibility of a concept, but would further review the roundabout operation with one of the specialized roundabout analysis methodologies. If the results suggest further analysis is needed, I would have the network modelled by one of the detailed microsimulation programs.
Multilane roundabout geometry is less forgiving of design errors than single lane geometry. I would make sure a multilane design was evaluated with a detailed microsimulatuion program and was reviewed by someone having experience in roundabout design.
