Since these Line 6 systems are all using 2.4 GHz, you should be assessing the RF environment in the location you are trying to use it EVERY time. You must run the channel auto-scan every time to look for clearest open channels. If you are running multiples Line 6 systems, you need to scan for each, leave it active and repeat the process and scan the next until each has the clearest open channel.
I don't see this stated in many of the comments from those complaining of drop outs.
Just checking if your iPhone or some other wifi device has clear signal is not going to help you and any active wifi device too close to your Relay is going to impact both. With the proliferation of wifi control of mixing systems, there are more devices competing with bandwidth. If you have other wifi devices iPad, iPhone that you use to manage your PA or monitor mix control, if you can, move them to 5.8GHz.
You cannot compare to a UHF based system or even Europe vs North Amerian without understanding how crowded the 2.4GHz is where you are trying to use.
I use my G75 professionally for 50 plus shows a year with two transmitters, I scan for open channels on both EVERY time, at every location (twice, once for each) 98% of the time I have no issues, but there are some venues where the 2.4 GHz is so crowded near the stage there is just too much interference to use reliably.
I don't understand why Line 6 doesn't make it clear that it must be done EVERY time and make the interface even easier to complete this process. I read that the Relay G10 scans for an open channel every time you start it to use. They all should do this. I have been asking that Line6 improve the interface on the G75 since the unit was released to make that the first thing you do every time you turn it on.
This is from 2010... and still applies.
http://line6.com/support/page/kb/_/live-sound/relay-digital-wireless/24ghz-wireless-faq-r242