I just completed a 3,500 mile 15-state road trip and used my ez-pass extensively. My transponder worked in some states and not others in this position. I eventually dismounted it and opened the sunroof every time I went through.
In Illinois and Virginia the lanes don't have gates, and they will fallback to photos if the transponder doesn't work. In Indiana or Ohio (I forget which) they have a barcode scanner that you can place your transponder in if it doesn't work (this gives you an idea how common failure is.)
In one state (Indiana?) the transponder wasn't read when I got on, but it was read when I got off. The result being I was charged the maximum amount.
Using the license plate frame transponder is not an option for me because in Virginia we have flex transponders that have a HOV selector on it to indicate if you're currently HOV. You can't do that with the license plate. I really wish Tesla would address this problem.