Well, I am an amateur leatherworker; I feel competent to comment (as you may have noticed, I feel competent to issue pronouncements on every subject that comes up on this forum ... except khukuries. What can I say? I know everything ... I can't help it ... except about khukuries; I know nothing about khukuries....)
Just about every leatherworker and every book about it says there's only one leather dressing that's any good and using anything else will destroy your leather. The only trouble is every one of them recommends a different leather dressing, and all the others say it'll destroy your leather....
I'm reminded of a theology professor a friend of mine had in seminary who would say, so-and-so says this, so-and-so says that, explain what all the authorities had said on the question -- and then at the end of the lecture he would peer at the class over his glasses and say in his thick German accent, "And now, gentlemen, the truuuuuuth," and then tell them what he thought about it. So ...
And now, gentlemen, the truuuuuth: if you never put anything on your leather it'll eventually dry out and crack, but you can prevent that by putting anything oily or greasy or waxy on it. Oils and greases will soften it, to varying degrees and according to how much you use. The best waterproofing is waxy stuff, and Sno-Seal is probably the best commercial boot wax. It doesn't soften leather. It doesn't look particularly great, but it's great waterproofing.
Nothing oily or greasy or waxy will somehow destroy your leather. If you want to destroy leather goods for some reason, try battery acid ... nothing else will do the job like battery acid ... bleach is good, too, soak it in bleach for a long time and it'll dissolve into a gooey mess.
Linseed oil will harden; it's the basis of oil paint. If you have a wooden scabbard or something that's covered with leather you can put linseed oil on it and it'll darken it a lot and when it dries (takes forever) the leather will be hard. Don't use linseed oil on leather that isn't covering something solid and inflexible, though, it'll get hard and break instead of bending.
Don't overdo it whatever you use; you don't want your leather soaking wet with oil or grease ... it's often surprising how much it can soak up, but don't use more than it can soak up.
Neat's foot oil is fine if you want to soften leather. The Kiwi products are fine -- I use the Kiwi shoe cream instead of dying leather sometimes. Kiwi wax, the can of paste wax, is a good choice if you don't want to soften the leather. Looks better than Sno-Seal but isn't as good waterproofing.
Getting wet once in a while doesn't destroy leather either (though rawhide dissolves in water ... most of the stuff sold as rawhide, though, isn't; it's latigo, vegetable-tanned cowhide that's been treated with neat's foot oil). If it dries too fast it'll shrink and harden, and getting wet washes the oil out of it, but that's all; water does not destroy leather. Use battery acid for that.
Neat's foot oil is made from the foot of a neat ... of course ... what'd you think it was made out of??? Actually it's made from the hooves and shin bones of cattle.
-Cougar Allen :{)