The Mutacrat Sub-Space Drive is a good example: the author explains why the species cannot use warp drive, and then just says they develop a different kind of interstellar drive it's simple and understated. Badly done, it's annoying, presses the bounds of canon(where it doesn't cross them entirely), and comes off as shallow posing. Well done, a fanon interstellar drive can move the science of WH40K to new bounds and open up new roleplay and storytelling opportunities. Only the "silver tongued" Imperial Inquisition gets Eldar tech and even then it's highly likely that only a few Inquisitors per sector have Eldar technology. But I should say, since these subjects are extremely light in detail for the most part, I recomend not making your xeno race have wraithbone technolology or the power to drain stars of life. Also should add that Cadia was once an Eldar world and still has gates on the planet and is actually used by the Imperium to much reluctance though. But for the most part, Eldar will never willingly give their weapons away. We also know certain Librarians of the Blood Ravens Chapter were given Eldar-crafted force weapons. But for the most part, Eldar are just as xenophobic as the Imperium is. Eldar have given certain humans their tech, only when trusted extremely or out of debt.
Whist the above said is true, there have been cases where Harlequins have given Inquisitors weapons and items. Even today, the Eldar will turn to no other speces to aid them in their hope of rebith in the galaxy. The Eldar brought about their own fall due to their own arrogance, and still uphold this Eldar tenet. This is why the Eldar have no qualms about killing humans as far as they are concerned, it is as much the same thing as is culling livestock (albeit livestock with guns and knives). If the Imperium view other xenos species with cold hatred, then the Eldar view the same species with absolute and utter contempt, seeing them as little more as we would cows or pigs. But under no circumstance can any race other than the Eldar have an affinity for their technology or use wraithbone psychoplastics. It can be salvaged, in the same way that Orks loot things.
So let's get this clear right now: THE ELDAR DO NOT SHARE THEIR TECHNOLOGY WITH ANYBODY. This is perhaps the most egregious, annoying, and most common canon-infringement I have seen.