What I found most jarring was the close button on the wrong side of the title bar, and the wrong shape. In Windows 3.x, Close Window was the last alternative and default double-click action of the System Menu over on the left, just as it still is in at least some apps. It looked like a big 3-D minus sign -- for all apps; not a unique app icon as now -- which actually, AIUI, was a skeumorphism for the space bar, which (together with [Alt]) is what you pressed (and still do in Windows 10!) to drop down the window-handling menu.
There was no Close Window button at all on the right end of the title bar in 3.x... So it took me the longest while to stop closing stuff by mistake when I just wanted to maximize / midsizify a window, after moving to NT4 which shared the W95 UI. Now, I seem to be used to it: I just had to close an app by double-clicking its system icon at the top left, to double-check that it still works that way, so apparently I don't routinely do it that way any more. (Hey, that means it took me less than twenty years to learn!)
Icons clearly come from Windows 95 (some, I think, from Millennium even). Not in itself a bad thing (I do actually think 9x icons were an improvement), but if it’s advertised as mimicking Windows 3.11, it better stick to that promise. Flat toolbar buttons are also out of place. The spacing of buttons on the title bar matches neither version. It’s also weird seeing the theme with Ubuntu fonts instead of System and MS Sans Serif. Scroll bars do look pretty faithful at least.
What I’d like to see someday is a theme with a kind of remastered Windows 3.x/9x look: icons upscaled, cross-hatch Bayer dithering smoothed out into flat, solid surfaces, dotted lines replaced by semi-transparency, bitmap fonts remade into vector fonts while maintaining all the character shapes, so that they maintain the look and feel while taking advantages of high-DPI, high-colour screen. That would be a joy to look at, kind of like watching an episode of Star Trek TNG in 1080p.
It's definitely a mix of the two, but I think the icon style, the window borders, scrollbars, fonts, and colors are all more reminiscent of 3.11 than 95.
There was no Close Window button at all on the right end of the title bar in 3.x... So it took me the longest while to stop closing stuff by mistake when I just wanted to maximize / midsizify a window, after moving to NT4 which shared the W95 UI. Now, I seem to be used to it: I just had to close an app by double-clicking its system icon at the top left, to double-check that it still works that way, so apparently I don't routinely do it that way any more. (Hey, that means it took me less than twenty years to learn!)