An ILS gets you lower than a VOR approach. Since the VOR approach will not get you lower than the ILS, the VOR approach is more restrictive than a ILS. Logically, if the weather (visibility low and low clouds---say 300 feet) the VOR approach would not work but the ILS would (typical ILS DA 200 feet AGL)
Reference KRDU: ILS 23R has the nonstandard alternate symbol. VOR 23L uses standard alternate criteria.
VOR 23L MDA brings you to 625 AGL. ILS23R brings you to 200 AGL but the nonstandard alt. is 800-2. So to file RDU as an alternate, the non-precision VOR approach allows me a lower ceiling(625 and 2400 RVR) than the ILS 800 ceiling and 2 miles vis. yet the precision ILS ironically calls for a ceiling 125 feet higher compared to the 625 foot ceiling of the VOR approach.
My question: Why have nonstandard alternate (higher mins.) for the ILS 23 R but standard alternate (lower mins.) for the VOR approach?