I don't understand why Split Horizon rules will cause problems on NBMA networks such as Frame-Relay Hub-and-Spoke for example.
No it does not mean that you can not use IGRP or RIP with Frame Relay NBMA. It does mean that you must disable split horizon to use these protocols (also applies to EIGRP). When you disable split horizon you have disabled one of the primary defenses these protocols have to prevent loops so you must be especially careful that the environment does not cause a routing loop.
OSPF does not elect DR/BDR in a point to multipoint environment because it does not meet the full criteria for DR/BDR. The DR/BDR mechanism is effective and increases the efficiency of the routing protocol when it is certain that all of the routers in that subnet have full connectivity and visibility to each other. In environments like Ethernet that is true and the function of DR/BDR can reduce the protocol overhead since routers only form full adjacency with the DR or BDR but not with other non-DR/BDR routers. In the point to multipoint environment it is not certain that each router has full connectivity and visibility to every other router and so each router establishes full adjacency with every neighbor without concern about DR or BDR.
There is a reasonably simple explanation of this. To illustrate the issue lets assume a Frame Relay hub and spoke with 1 hub configured as multipoint (3 PVCs on a single interface rather than point to point subinterfaces so it will be NBMA) and 3 spokes. Spoke1 has 192.168.1.0 on its LAN and a Frame Relay connection to the hub and advertises its LAN to the hub. Spoke2 has 192.168.2.0 on its LAN and a Frame Relay connection to the hub and advertises its LAN to the hub. Spoke3 has 192.168.3.0 on its LAN and a Frame Relay connection to the hub and advertises its LAN to the hub.
So in that environment the hub has learned 3 networks (192.168.1.0, 192.168.2.0, and 192.168.3.0) on its NBMA (multipoint) interface. Split horizon says that you do not advertise out an interface routes that you learned on that interface. So the hub does not advertise these networks to the spokes. And the result is that none of the spokes learns the networks on the LAN of the other spokes.
That is the essence of the problem with split horizon with Frame Relay multipoint.