Disclaimer, I worked at Netflix for 5 years. There are some interesting challenges in the clients. The TV client needs to run on devices with very little memory, and processors roughly as powerful as a Pentium 2. The JavaScript must be interpreted too, no JITing allowed. Getting smooth experiences everywhere in the world where internet quality and device power varies drastically, all in one code base that about 100 people work on, is a pretty good challenge. For example an entire proprietary "browser" was created with the goal of being low memory and high performance. So for TV devices, the entire dev stack from top to bottom had to be created mostly from scratch. This included forking React to create a version that would run on this platform.

It's also interesting on the other end, what cool stuff can be done on the PS5 for example? Shaders and animation techniques are also pretty interesting to explore.