The issue with Uplay is the same as the issue with EADLM...which evolved, eventually into Origin.
For those that remember, EA had a downloadable content service in place for a long stretch previous to Origin. But previous to Origin, EADLM was not an exclusive content store front, it was simply a way of delivering content to a user, regardless of how they had purchased it.
Origin grew out of EADLM, and became an exclusive store front where all games produced by EA eventually got stuck behind it and the only way to access those games is by using a different digital content delivery system than the one you may have all your games on already (In most cases for PC users, that would be Steam).
UPlay is already several steps ahead of where EADLM was, and while its not an exclusive content delivery system yet, youc can tell Ubisoft wants to go that direction because instead of simply accepting content authentication through Steam they want that next layer of authentication by requiring Steam to launch UPlay and then have your game content authenticate there.
So overall its a needless complication of systems which leads eventually to exclusively gating content based on forcing users to only use UPlay for certain titles. Its really where these "digital storefronts/content delivery services" want to go.
Eight years ago these very companies were pushing hard copy ownership and subsidized/co-opted Gamestop to attempt to market PC gaming into an early grave by making physical game stores arrange their stock display to seem as if PC games were irrelevant (PC games usually ended up stocked on one back shelf, usually sharing one side of a two sided shelf and the other side almost ALWAYS had console accessories on it, and that was the side facing the store entry, not the PC games) and console games were what everyone was buying. This was their gambit to control gaming, through console DRM and the highly fabricated illusion that PC gaming was a dead genre....and the people that were spearheading that charge? EA and Ubisoft...the usual suspects.
Once they lost that particular argument, largely because Valve and Steam proved them demonstrably wrong on all counts and PC gaming was actually surging, not slumping, they cut their losses at Game stores though they still lease huge swathes of shelf space in major game retailers to this day, but then focused their efforts on trying to control access to their titles through their own DRM systems....to combat piracy, or so the party line goes. But in all fairness these companies lose more money to internal overhead and production waste than they'll ever lose to piracy, piracy is a hugely convenient boogeyman that gives under-performing titles an excuse to pawn off to investment boards in regards to why the title possibly made less money than it could have made....and being able to control the point of access to that content, or the authentication of it, allows a company like Ubisoft or EA to reliably manufacture their own metrics to support their arguments.
Also, its just annoying that everyone needs their own CDAS solution these days. I mean, no one says that Steam and Only Steam may be our god of "Deliverance" (har har, smarmy pun)...but why does a company with a catalog of PC titles that is small enough to be counted on the fingers of one hand...for example, need its own CDAS solution? Simple facts are is it doesn't and just letting Steam do what Steam already does would be more than acceptable. I find it to be unnecessary waste of CPU cycles to have to run multiple content delivery/authentication systems, simply to appease some other billion dollar failures board of investors.
So its not that Steam was fantastic but at least it grew into what it is today by doing solid justice to the community it attempted to serve, the CDAS systems that have come up behind it are late to the party, and aren't just trying to sell you games, they're trying to control your access to games...and that is an extremely concerning point in the difference between Steam...and anything else.