8 queries implies 8 tables, which is well under the limit for both Postgres and MySQL where the planner may give up and choose a suboptimal plan.

I'm not sure thats true for Postgres. optimal join ordering is np-hard, and finding an optimal join requires exhaustive search through n! combinations (n=number of joins) - thats why postgres generally uses heuristics to figure out join order. 8 is also the default value of "join_collapse_limit" setting in postgres, so it can't ever reliably optimize over 8 joins at a time. Additionally, postgres starts using "genetic algorithms" aka testing random combinations of joins with 12 joins by default (geqo_threshold setting).

I generally agree its better to use database to its fullest, but I would say 8 joins is probably the "limit". Internally at work I've advised teams to try to avoid anything more than 6 joins for "hot-path" queries.