I noticed some of the reviews of the ACR Res-Q-Link PLB indicate that the beacons "must have a clear view to the Southern sky" for it to work. "It must be a clear view to very low in the southern sky, (like nearly to the horizon). It didn't work in an even slightly wooded/forested area."
In those reviews did they use the Test button or the SOS button? If the latter, they must have made some type of prior arrangement with the PLB response authorities to avoid triggering a rescue effort. That makes me suspect they only used the Test function, which I addressed above.
There's another layer here that I think a lot of people overlook. When you register your PLB with the government, you provide a primary contact to them. This person will be notified if your PLB's SOS signal is detected, even if the GPS location did not make it through. Before every trip I give my contact person a very detailed trip plan, including a topo map screenshot. I use MS Excel and Nat Geo TOPO to generate this document, so I give them a hardcopy and I email them a PDF file of the document. This way they can easily email the PDF file to authorities hundreds of miles away.
If I send a PLB SOS and the GPS function doesn't work, by getting in touch with my contact person the SAR effort will still get my full trip plan and map. This goes a long way to backing up the PLB's GPS function.
Another thing: the PLB's power is 5 watts, which dwarfs the SPOT-2's 0.4 watts (400 mW). Plus the PLB uses a lower frequency than the SPOT-2, and the lower the frequency the better the signal gets through tree cover.
Bottom line: you can dream up all kinds of scenarios where a PLB might not work, but it's going to work better than you think and it's going to work better than a SPOT.