Why are end-fire glideslopes less useable for low-minimum approaches than image-type glideslopes?
End-fire CAT III is available.
A 1990 paper[1] mentions that the slotted cable end-fire system is able to provide the highest levels of performance for any runway. One of the references is the WAC Model 105 from 1986. And indeed the product page confirms the CAT III option. I was also able to find the Leonardo Model 2100.
But, there's truth to what @UnrecognizedFallingObject wrote, at least for the US.
A 2008 assessment[2] of how the US evaluates the G/S performance has found that the current methodology does not account for the end-fire signal structure, the requests for waivers is in most cases declined, the rewriting of the standards would be too costly, and for the FAA to &quot;have the latitude to apply alternative analysis methods&quot;. The assessment also reached out to Rockwell Collins, an auto land manufacturer, to understand how auto land systems utilize the G/S, since information about this is very limited (commercially sensitive information). And it turns out past the threshold the auto land system does not use the G/S, which is in favor of the end-fire evaluation issue – &quot;In other words, the specific computational method used to estimate RDH is not important as long as it is representative.&quot; [bold emphasis mine]
Of note in that report from 2008:

As of this writing, all endfire glide slope systems operating in the U.S. are certified only for Category I use.


1: Watts, C. B. &quot;Applications of slotted cable antennas in the instrument landing system.&quot; IEEE Aerospace and Electronic Systems Magazine 5.5 (1990): 16-20.
2: Edwards, Jamie S., Michael F. DiBenedetto, and FAA Aeronautical Center. &quot;ASSESSMENT OF THE EFFECTIVENESS OF THE RDH/ARDH EVALUATION METHODOLOGY FOR THE ILS GLIDE SLOPE.&quot; (2008). (PDF)