UK5 Eye of the Serpent was not 'bad', certainly not in the way that The Forest Oracle was just plain bad, and there were far worse than UK5 available (A3, B3, B9, X3, C3/4/5 the whole DL series, I10/11/12/13, most of the H series - I;d much prefer to run UK5 to the Temple part of T1-4 even!)
UK5 attempted something different - a wilderness trek for a small party of 1st level characters. There were no editing issues, the presentation was fine, it was easy to use, it made sense, the encounters worked, and it described the changing environments as the party trekked down the mountainside very well. But without much of a plot, or any hook better than 'ooh look, that river looks like a snake and it's eye is glinting', it just felt very average. It needs a spark, some sort of purpose, it lacks personality.
As for the railroad elements - that was easy to avoid - remove the 'open/closed route depending on classes' feature. My group played the adventure a couple a months ago when we started our 1E campaign - I gave them the freedom to roam, assumed all routes were open, and they had a good time. Not a particularly memorable one - nothing on a par with 'crazy frenchmen (X2)', or 'terrifying Monks (X4)', 'or the Web explodes and fills your minds with clear visions of others' lives (UK4). Merely good.