I was also stuck at Google Boot screen today, a few days after finally flashing 5.0.2 on the 4G version. I had to power it off because it was not recognizing the SIM (has happened twice in the last weeks), so I get the SIM out 'cleaned it', put it back and power the tablet on. Stuck on Google Bot screen. When going into recovery mode (TWRP) the touch screen was not working so I couldn't even wipe or recover the flash. In bootloader mode the volume and power keys worked but it was not very useful, I could just power off the device.
Then I read the comments about 'loose' things in the back of the motherboard and I tried just pressing the screen and the back of the tablet with extra force, without even opening it, just to see if it could work again (kind of old style hitting CRT TVs to make them work
). After a few attempts in TWRP recovery the touch screen was finally responsive. I tried to just boot to System and it's back working ...
So maybe this points to bad construction by part of ASUS and stuff is going loose inside the tablet and stops working making the firmware boot to get stuck waiting some response from some unresponsive hardware. I just wanted to share my experience in case it helps other. If the problem resurfaces I will come back.