It's never really been an issue with respect to charging a Class A reservists or compelling him to attend a trial. Class A reservists have been charged and tried before summary trials (under the old system) and courts martial for decades. I've prosecuted a few myself. I've never heard of any Class A reservist refusing or failing to attended before a service tribunal as ordered/required.
The problem is that the military complicates things (unnecessarily at times IMHO) and the administrative processes required to generate all the steps for a hearing or a court martial are harder to do in a part-time organization than a full-time one. It's the same challenge but more difficult to execute the processes.
S118.1 is not new. It has been around in various forms for quite some time. In addition there has always been a power to arrest an accused. The recent change to s 118.1 has to do primarily with the removal of the term "service tribunal" from the NDA and replacing it with the terms "judge" and "summary hearing." The intent with the last amendment is to turn the "summary trial" into a "hearing" so as to be more an administrative action rather than a judicial one.