Series resistors are usually included in the emitter assembly. There could be some minor differences here, possibly resulting in lower output. If the emitters are pasted over the equipment's IR sensor, I don't think that these variations would be an issue. I can't recall any examples, but I can remember a seeing an emitter that used a 3-conductor plug. Inserting a 2-conductor plug into a jack that expects a 3-conductor plug could result in issues.