It offers excellent scalability abilities.

I would rate the scalability an eight out of ten. We currently have over six clients for this solutions. 

Scalable to millions of users worldwide.

We have only enterprise customers.

I am not aware of any problems. All system upgrades went smoothly from an administrative perspective. I believe the Server Engineers could say the same if you asked them. My observation over the years is that the system upgrades can be complicated and must be well planned out to address 'what if' issues, including backing off upgrades if necessary.

I have found RSA Authentication Manager to be scalable.

No concerns from the product on the scalability.

It is scalable. We have about 100K users. 

We have encountered issue when trying to expand this particular solution for a large set of users across the country.

No.