Benitez has wasted money on some pretty average players over the years,Yes.No is doubting that.A lot of players that you mentioned in that list were given a proper chance,but they never fired.He has had to play players out of position,like drifting Gerrard to the right etc.But that does not make him a bad manager.
Being a poor transfer-man is a valid argument against him,but what people ignore is the fact that HE had control over who he wanted in England.In the EPL,the coach decides who he wants etc.In the Serie A,it's different.It's the board that does so.Benitez won't have so much liberty as he did at Liverpool,so the chances of poor players being signed would come down.
Do keep in mind,that there's a difference between a signing a poor player,and a good player not hitting form.People end up putting both of these in the same basket.