Again, Frieza is the same person that destroyed planet Vegeta because of a legend. He is established as a coward who is scared of the idea of someone being better than him and lashes out with excessive force to eliminate that threat. Then some Saiyans start causing problems and he plays around with them, sure he's a sadist but continuing to hold back against some of the strongest opponents he's faced when he knows about that legend is as much a case of poor judgement as Cell not believe that Gohan could actually surpass him.
And I have to disagree with Cell being boring, I find him far more engaging in his actions throughout the arc and his personality to be very interesting compared to other villains in the series. But that's just my opinion, I'm not trying to make you like Cell, however your argument for why he is not a good villain is lacking and can be applied to others in the series.