Guinsoo's is a confused item that tries and fails to do a lot of things at once. It's been designed and redesigned multiple times to be the go-to hybrid AP/AD item, in an environment where there is no relation between the two stats, and the result is that it needs to be far too strong if it can hope to bridge that gap. In addition, that kind of hybrid gameplay doesn't really work well with its other niche as a scaling item for on-hit champions, and the two niches would likely be better off being catered to by two separate items. At the end of the day, Guinsoo's is an item that tries to insert multiplicative scaling for two completely different playstyles that typically lack it (i.e. on-hit builds and AP/AD hybrid builds), and because of it puts itself in this awkward position where it's either crap or far too strong.
At the very least, it should get split up into two items, with one multiplying on-hit and the other enabling AP/AD hybrid builds, but even then I doubt that would produce good gameplay. I very much agree that the core mechanic to Guinsoo's is a win-more effect, rewarding champions fed enough to attack 6/3 times and survive with more persistent power, while providing no real benefit to champions who are behind and can't afford to stay that long around their targets. I think it might be time to realize that on-hit builds aren't really capable of generating interesting gameplay on their own, mainly because they mostly just operate by providing more raw damage (which AD does just as well), and that hybrid AD/AP builds need to be supported directly onto champion kits, rather than forced in through item purchases, if it is even possible to do so at this point. In the meantime, it might be better to get rid of Guinsoo's altogether.