For low level work though -the mustang was particularly vulnerable with its cooling system design/layout
No more so than any other aircraft. All the liquid cooled aircraft were used for low level work at some stage, chap I worked with as an electrician had his brother, Ian McRitchie, shot down by flak in a Mosquito during Operation Jericho. Radial engines were not immune, a single round through the oil tank/cooler could occur as easily as a round in a liquid cooling system. The first time I was shot down was by a single AK-47 round in the engine oil cooler, engine (T53) ran for about two minutes following loss of oil pressure, which was instantaneous with the round hitting.