Nothing is stored on the visitor's device, by default. PostHog uses cookies unless cookieless mode is switched on, and when it is, it identifies visitors with a daily server-side hash, which is the same technique Skomi uses. The difference is which way round the default points, and a default is what most installations keep.
There is no instrumentation project first. PostHog rewards a team that models its events carefully, and it depends on one. Skomi's page views, sources, goals and funnels work from one script with nothing named in advance.
It is a smaller thing to run. PostHog's breadth is real and it is also surface area: more products, more settings, more to configure and to keep configured.