Fans hate him because he played at a small school and statistically wasn't anything special. Seeing a guy like that ranked, in a lot of cases, ahead of guys like Darnold, Rosen, and metrics-darling Mayfield rubs a ton of fans the wrong way.
Allen is very much a projection; he probably has the strongest arm we've ever seen and tested out in roughly the 82nd percentile of NFL QBs athletically speaking (basically Carson Wentz level athleticism). And he's a classic late-bloomer; Wyoming was his only D1 offer and they only offered him because Eric Dungey came here. He's received basically no coaching, which has led to some issues with his footwork (mainly laziness where he relies entirely on his arm strength rather than setting his feet to throw), so people feel that there's a lot of room for growth with him.
And it's simply hard to tell how much of his play at Wyoming should be blamed on himself versus his teammates; his O-Line was so helpless that there were numerous examples of guys literally not blocking anybody, basically looking like that scene in Remember the Titans when Sunshine tells his O-Line to let that one pass rusher through. His WRs had some brutal drops, but for the most part caught the ball fine; they just couldn't get open on a consistent basis.
Watching Wyoming games made me want to pluck my own eyeballs out half the time.
At any rate, Allen is an incredibly talented guy, but he's a very major risk.