Most of the transformerless mics that I don't care for, which is the great majority of them, has less to do wit the lack of transformer, and more to do with other aspects of the design. And in some cases, the impedance converter used in lieu of the transformer may be a culprit, but I suspect it has more to do with the capsule, amplifier and filter circuits.
The other thing to take into account, that I don't think anybody has mentioned yet, is that in tube mics, transformers are made specifically to work with that tube, and the tube and capsule have a symbiotic relationship, as well. There is a delicate balance. One of the reasons that many substitutes for the VF14 made the mic sound like ass has more to do with the tube not pairing well with the transformer, and less to do with the sound of the tube.
Now, as far as getting a flat frequency response out of a mic, for musical purposes, I can't imagine ever wanting that.