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On one hand, the fact that the RNG algorithm is placed in the idle loop can make it very hard to manipulate its value, because the length of this loop will be affected by how many things the game has already processed on that frames, where in the loop the NMI interrupt happened on the previous frame (since it will resume exactly where it left off on the next frame), as well as CPU/PPU sync (which is deterministic on most emulators but random on a real console).<br />
On the other hand, this RNG algorithm is extremely prone to getting stuck on either a single value, or a very limited set of values. In other words, the idle loop will pretty much always loop over a limited set of valuevalues. There are even values from the BonusTable array that are never going to get picked.<br />
On top of that, the RNG value will always reset every 16 frames (to value 0x7A). This gives the value that the RNG will assume a degree of predictability.
