Too-simple RNGs can qualitatively change games. Example: In Final Fantasy 3 (SNES), a certain item was supposed to be obtainable, but it required RNG sequences that never occur.

Incidentially it still happens in modern MMOs when the developers forget to put items in a boss loot table. (Can't black-box QA a 1/1000 drop!)

Makes you wonder if something like Estamel's Modified Adaptive Invulnerability Field is rare because the loot tables are broken. (This is an Eve online item that is rarely seen. Only two ships have ever died with one equipped at least as of a year ago. Its rarity is such that it's worth 24B ISK -- or the equivalent of $2,000 in ingame currency.)

Maybe the ships don't die with it as they are invulnerable?

If 3rd parties can build bots, then they should be able to too :P

Surely a unit test of the code would find such an error?

Ive never quite understood that stuff. Monster Hunter, Fire Emblem, etc, all have seeded RNGs. In some cases, with fixed tables to make it even worse so that grinding for something for months will never get you what you want without restarting in some cases.

Are those devices really unable of anything better?

If the hardware doesn't have a clock that keeps track of the current time, the game could still re-seed based on the timing of player actions, because there's a certain amount of unpredictability there (at least, more than "seed with 0, only iterate on easily-controlled player actions"). It could do something like iterate the RNG every frame that the player doesn't move.

One of the reasons rare drops exists in RPGs is to arbitrarily extend playtime by hours.