Specialist is too thin. The specialists only know what they know and they don't want to learn new things outside their speciality. So I had to hire a new person everytime we found a speciality gap because the specialists were like "not my job, I am an X specialist, go hire a Y specialist". Then, they held their work tightly, no cross training, so the specialists all became their own brand of bottleneck. Different work speeds and different levels of quality meant that ego came to defend against performance complaints, and I as a manager couldn't add more people to the problem areas because they weren't trained in that area and the specialist could do it faster than they could train others to help.

That being said, all my full-stack team members had specialities. That's what T-shaped means. I had frontend specialists who could work the whole stack, backend specialists who could work the whole stack. Dev tools specialists who could work the whole stack. Architects who could work the whole stack. Everyone we hired had something they were best at, and an alignment to learn the whole stack. Within a year they were able to work on all tech in the stack and anyone could bring in a new tech to solve a problem and everyone would learn it.