The fact that Ian Book's arm was barely average, and couldn't make a reliable long range throw was the biggest criticism. It's not wrong to put forth those criticisms.
He was still the best man for the job, since he could run about 75% (my estimate) of Brian Kelly's ridiculously complex playbook, and still effectively spread the field. What we saw in his time as a starter was how effective things could be if you had a quarterback who could do most of the criteria. Against almost anyone, that's enough to win.
That was certainly good enough for two playoff appearances, and two decent bowl victories (Citrus, Camping World), but we weren't going to beat Alabama, Clemson (when healthy), tOSU, etc. with that level of performance. We needed a lot of help from Lady Luck, as well as Clemson missing most of their NFL drafted front seven in 2020 to beat them, and even then it was by the skin of the teeth. The rematch in the ACC Championship game proved that we were still not in that absolute top echelon.