The problem is that there are many 4-5 star flops in the transfer market, fast trainer or not, expensive or not. Usually, there are bidding wars for 4-5 star players. Buying them only to find that they are flops (I call their profile fake) is big waste of tokens. You can still time your bidding and get some 4-5 star players like I do with minimum tokens spent. But still, many can turn out to be flops. For me, a 5 star rated player, fast trainer or not, providing a performance of 6-7 is not good enough, especially when you noticed that your match results were unconvincing and he was the one that had the poorest ratings among the players that played.
How about promoted academy players?