I mean, don't get me wrong, you CAN develop phylogenetic trees based on traits. That's how Linnaeus and all subsequent taxonomists did it before they could just look directly at DNA. But it takes a lot of traits, and has been preempted by DNA based methods anyway.
What about the rhyme “Sedges have edges, rushes are round, and grasses are hollow right down to the ground?” You’re telling me scientists are now saying that DNA testing is more accurate than just using the rhyme?
For example, if you looked at just the ability to fly, you would group birds and bats on a same group. But that would be wrong because bats are clearly more closely related to flightless mammals than they are to birds. This becomes obvious if you account for more characteristics of the animal (presence of fur, bone shape, pregnancy, etc) or if you examine the DNA.