One problem with this game is that doing anything in the game costs the same resource [oil], so some things are always going to end up being an objectively worse uses of that resource than others. The only reason there can even be a debate on whether running Oci or building tanks in the factory is a better use of oil is because we don't have access to the actual numbers there.
We DO have access to the numbers for PoC and expeditions though, and the numbers are... crap.
The cost of running expeditions is around 350 oil just to get the map and then 150 to run it. So you're using a bare ass minimum of 1000 oil, 5000 currency and 5 cores to build a Rascal. 12k Xp for that kiiinda sucks if you consider you're losing out on all the stage five/six drops that oil could buy. Emblem expeditions are an even worse trade. Well, I'm also never spending cores on frivolous crap again after missing my chance to grab that booster tank on Christmas. It's not 10 cores, it's 1/200th of a PT-76! (I'm also never spending cores on anything that says it has a '40% chance' of a five star tank because it clearly fucking doesn't.) The main benefit of expeditions so-far is that they've improved the oil per reward ratio of core/blueprint invasions.
PoC starts out giving you about one blueprint per 6 oil and then steadily gives worse and worse returns. By the time you start seeing bosses you're spending 35 oil per blueprint, at which point you may as well be using the oil at the factory as far as blueprints are concerned. Mid tier core and candy rewards for PoC... exist? It's hard to justify spending the time and oil on them though. Maybe it's better returns at high levels but by where I'm standing it's pretty bad.