I'm not sure, but maybe it has something to do with the size of the crystal? Our low-pressure spec with LIF is all small crystals, and our two LLIFs are our high pressure spectrometers. We don't use it that often (it's usually Tap/Pet depending on our needs), but when we do I haven't noticed any particular problems other than the typical ~50% reduction in count rate (well, 50% for Fe Ka, I don't know that I've cross-compared anything else besides the verification line).