I think their voices need to grow on you, for me it was the same.
I just needed to find a song that fit with keys voice very well and the same goes for taemin and since then, i feel like something would miss without their voices
i have known SHINee since their early years, Jonghyun's and Onew's voices have always been my favorites in the group. besides, i don't know if it is just my imagination but i feel like Taemin's voice regressed in the past few years, it is especially apparent when goes high. he sounds fine in his lower register though.