I don't have the game myself and haven't been collating the posts from people who have. Perhaps evidence is too strong a word at this stage, but the overwhelming impression I've gotten is that the game has all kinds of technical issues other than performance, and given Ubisoft's track record, I'm more inclined to go with Unity being rushed rather than too demanding for today's hardware.
Actually, there is a lot of evidence that the game is actually pretty well optimized for PC.
I think people forget a few key things when talking about PC optimization.
1) Game was made with Xbox One as leading platform. Therefore, the game was made with 8Gb shared RAM target. Your PC cannot have that and this game pushes so many textures at a time, even at low you eat 2Gb. This is okay for new-gen consoles but not for older graphics cards.
2) Game is not being downscaled to match the demands of lower-end PCs. Watch Dogs was made so that despite all the "next-gen" city talk it could be run on PS3 and X360. Therefore, the underlying mechanics could be scaled to fit 128Mb ram of PS3 at worst.
Unity, on the other side, could not be run on last-gen and no development effort was spared to downscale anything for older hardware. Can you imagine the task of Kiev team, when they receive next-gen console code and need to optimize it for older PCs? It's not like Total War series which is made for PC only and has a slider where you can adjust unit size.
3) This game is the best-looking game on PC. If you run into a tree in Crysis 3 you can also make it look like ass. In motion, Assassin's Creed Unity is insane. It doesn't push millions of blades of grass killing the framerate, instead it pushes people. This time, it pushes 10000 NPCs even if you're sitting perched on top of the Notre-Dame cathedral. The LOD on those is insane.
This game is also running like ass on consoles. Compared to PS4 and XboxOne, you can run this game almost maxed out at 60FPS with midrange modern Nvidia card without SLI.
4) People here clearly underestimate the potential of new consoles and overestimate the power of PC. The Unified RAM for example is something which doesn't happen on PC. You all wanted next-gen games, without any links to old consoles to "provide better experience" and "bolder vision" and here it is. Unity doesn't compromise. It is given 5Gb of RAM and it fills 3Gb with textures. It doesn't dial down the number of NPCs. It doesn't downscale the city size. It has a huge amount of sophisticated animations. It also runs at 900p barely at 20-30 FPS on modern consoles.
I think that the results we're seeing from PC hardware are more than reasonable. People are able to run this thing at twice the FPS with 50% more resolution, better textures and effects on mid-range 970GTX and an i5 processor from 2 years ago. Nvidia cards perform admirably, AMD cards may have a bug, but this happened before.
And yes, this is the Crysis 3 of next-gen. It doesn't care for performance and pushes its vision to the limits. Crysis was just the same. It ran like shit and even now you can barely max it out. Yet it's seen not as a failure to optimize, but as a benchmark for PC performance, even though we all know that you can do the Crysis-like graphics and make them run better.
There are bugs in this game. There are issues. It may not be fully optimised, but it performs admirable considering the source. All I want is to say that guys at Kiev did a good job.