Good question. We used to have UI Engineers dedicated to each engineering team, and they were doing almost all of the frontend work for that team. As you pointed out, they became a bottleneck. The other problem this created was that engineers actually felt _disempowered_ to write HTML and CSS — even if they were a frontend engineer. The UI Engineers were good and trusted by the engineers, but so much so that they learned to rely on them and didn’t want to write any HTML or CSS.
In the past ~2 years we’ve evolved so that the UI Engineers don’t work on features anymore, and aren’t dedicated to any engineering team, but instead focus on building our design systems that make it easy for anyone to quickly build UIs by using re-usable components (coded in React, in our case). The UI Engineering team is smaller now, and scaling well across our engineering and design teams.