1) You were contracted to install 2 TVs? Did that include installing the wireless DirecTV clients? If so, then I would have had the conversation about how flaky those boxes are and may not work consistent & reliably. If coax was available at the TVs (or able to fish coax to the locations), then I would have given a quote on that and recommend replacing the wireless boxes with regular boxes.
If they choose to go with the wireless clients anyway, then not my problem if they flaked out after a proper install - customer was warned. To quote that grand YouTube video "F^ you, pay me".
Really... this is a DirecTV problem anyway. Customer needs to call them after they write me my check for installing the TVs. End of disscussion unless they want to pay a lot of money for me to help fix their 'DirecTV' problem.

2) You can NEVER give an accurate summary of what to expect from ANYTHING wireless. The world just doesn't work like that. Some things and brands are much better at wireless than others, but there is NEVER a guarantee of performance.
I have seen those DirecTV wireless boxes work 80+ feet thru walls from the wireless bridge before. I have also seen them flaking out at only 20ft away. They use WiFi channels, so interference is a real issue in crowded areas.