Lightning took out Q90 on Mike K5ESS µBitx. It also fried the switching power supply he was using. Others have reported similar frying of Q90 due to lightning.
Doug WA8UWV suspects any strong signal or static pulse on the antenna during receive will damage Q90. He has another ham 2 houses away and DX Engineering is also located near by. He contemplated a reverse connected diode across the base and emitter of Q90 to see if it eliminated failures.
Gordon KX4Z has speculated before that the failure mode is a REVERSE voltage on the base-emitter, as the series capacitor charges up due to rectification by the base-emitter junction. This document on pp18-19:
http://cdn14.21dianyuan.com/download.php?id=170219
seems to suggest that indeed, overloaded reverse voltage bipolar junctions fail SHORTED, while forward current would be expected to OPEN them. The reverse diode would eliminate the charging of the series capacitor and protect the junction against reverse voltages, and someone tested this for ill effects and found none.
Gordon has put the reverse diode on his µBITx and similar radios and has had no failures as yet.
My ubitx was working perfectly until I turned it off and fired up my TS430 at about 60 watts on an antenna about 10 ft away from the ubitx antenna. When I turned the ubitx back on it had no RF output. I traced the failure to emitter base short in Q90. We need to disconnect ubitx antenna or apply other measures to protect Q90 if there will be nearby strong RF. The mixer diodes do not save Q90