Well art assets in general as bought off the self should be considered as a raw starting point as the odds of getting just the right thing for the right purpose is slim next to none but the one thing Unreal 4 does exceptionally well is what you have can easily be improved on via the iteration process.
Being an animator and in context a video game animator you can not tell until you see it in action if a given clip will preform as to expectations as it’s not usually the cycle it’s self that sells the movement but rather how the movement plays into and out of the cycle and would be a lot of work if all was done by scratch were tossing in movements as a proxy if just to hook up the asset chain than one can patch things up over a coffee and a muffin.
As an animator my single valuable resource is my MoCap library that is extensive with clips I bough as well made from scratch that I can select a number of different clips like paint off a pallet and using Motion Builder can tailor to taste and replace on the fly.
Overall if you have an application like Motion Builder that can batch edit making matched sets then what is outputted is of your own making even if you used off the shelf as the base.
P.S. Even Disney recycles their animations. 