Even if the data traffic between two points rides within the CIR, there's no guarantee that it won't be discarded in the event that the network becomes congested. And there certainly is no guarantee that the level of latency won't vary wildly and unacceptably as congestion develops from time to time. Remember that Frame Relay is a shared network. After all, apps are what it's all about. LAN traffic, by definition, expects nothing in terms of QoS, and Frame Relay offers none.
Performance is good and cost is low.
Frame Relay is great for the applications for which it was intended. With a little tweaking of network CIRs and timing considerations at the host level, synchronous traffic such as SDLC can be supported nicely, as well. Network performance is less than SDLC expects, but the cost is so low that Frame Relay is a great option here as well. A huge percentage of such traffic travels between customer premises and ISP, and between ISP and the Internet backbone over Frame Relay. Remember that Frame Relay is an access protocol for access to a packet network.