So the last time I looked I only found one person complaining that their ER-X runs hot to the touch (plenty of "it seems kind of flakey" comments tho). Mine become unstable once the ambient temp gets much past 75 °F. The ER-L is well known to run hot to the touch at idle.
When I opened up one of my ER-Xs to get a the serial header I noticed that it had a much sloppier application of thermal compound than the one pictured in the instructions I looked at. I'd totally believe some units will be more reliable than others.