It is definitely not good advice for all things. For workloads that are either end of the CPU/IO spectrum (e.g. almost all waiting on IO or almost all doing CPU

It is definitely not good advice for all things. For workloads that are either end of the CPU/IO spectrum (e.g. almost all waiting on IO or almost all doing CPU work) it can be a huge win as you can get very good L1 cache utilization, are not context-switching and don't need to handle thread synchronization in your code because not state is shared between threads. For workloads that are a mix of IO and non-trivial CPU work, it can still work but is much, much harder to get right.