Which of the following is a key characteristic of SoapUI?

The key characteristic of SoapUI is its capability to perform load testing of web services. SoapUI is a versatile tool that support both functional and non-functional testing, including performance testing, which encompasses load testing. It allows users to create complex load scenarios and simulate multiple users accessing web services simultaneously, which is essential for assessing how a service performs under stress.

In the realm of web services, load testing is vital for identifying performance bottlenecks and ensuring that services can handle expected traffic levels without degradation of performance. SoapUI's ability to facilitate such tests makes it a valuable asset for integration architects who need to ensure system robustness and responsiveness.

Other options present constraints that do not accurately reflect SoapUI’s capabilities. For example, SoapUI is not limited to functioning only with REST services, nor is it limited to functional testing or to simulating SOAP services exclusively. Instead, SoapUI supports both SOAP and RESTful services, along with a wide range of testing functionalities. This flexibility makes it a comprehensive tool for various testing needs beyond just functional or specific service type testing.