Having logs and storages share a process is a bad idea - however, having log and storage processes sharing a machine is completely fine (we do that for years now on most of our clusters, on the ones we didn’t the motivation had more to do with different VM-types on AWS which gives us more flexibility).
The main reason for not having them share a process is CPU: each fdbserver process is single threaded. But log and storage can both consume quite some CPU and if a log process is starved for CPU (for example because you run some range queries) your commit latency will go up which can have all kind of undesired effects.