I think the biggest challenge is one's own expectations of themselves. As a senior engineer I'm used to having a good handle on all things going on in the team, and that would traditionally enable me to help anyone on the team. With this many experiments and this many codebases I find that seniors tend to end up being specialists in various areas of the codebase. This does make me feel sometimes inadequate, but I also try and remind myself every time it's simply not feasible to have a good handle on everything and accept the unavoidable realities of that, and expect less of myself.