1. Sofar we have architected and migrated/implemented on MPLS VPN (either L2 or L3). Reliability has not been any concern to any of the customers. There were some problems but during the initial stages only.
2. Main issue I had seen is of IP addressing, when you are gong for a L3 VPN. Other issues are how well is your ISP in service provisioning and how well there network converge during any outage as major issues have been LDP failure and IGP/BGP cpnvergence.
3. You should know which falovor you should have L2 or L3. You should know your network well and should be felxible for IP addressing.
hope this helps.