Caution should be advised using glidepaths of less than 3 degrees, with large wide-body aircraft.
The threshold crossing height will be lower, and the main landing gear might well be rather close to the frangible approach light bits, as a result.
Automatic approach/land ops especially.
The requirement on reference datum height remains unchanged as the GP angle changes. For CAT II/III you're guaranteed your 15 m at the threshold. For CAT I, it's "just" a recommendation from ICAO.
For short CAT I runways, the 15 meters shrink to 12.
In a friendly note, ICAO points out that the assumed maximum vertical distance from the GP antenna to the path of the lowest point of the aircraft is 5.8 meters. More than that - beware.
Reference: ICAO Annex X.
I visit an an airport where they have a 2.86 degree ILS at one end and a 3.25 at the other every once in a while. Multiple approaches on the 2.86 degree followed by a direct switch to the 3.25 makes for an interesting change of perspective!