So yes, this would have needed a beta test, and there wasn't really one.
Having a bunch of "looking to break it"-minded players involved before release would probably have been sensible.
That said, I wouldn't have expected that to find everything, and it might also have come back with false positives (in the same way that the streamer advertising may have resulted in "Hostile" being toned down a little too much [1]).
- Stronghold Carriers being too vulnerable: yes, it would probably have found that; that said, I remain unconvinced it's actually a major problem [2] as opposed to something which needs a little adjustment to balance - not every Stronghold system gets a carrier, so a Power creating many Strongholds can have most of them be carrier-free; it stops being effective after the Stronghold is reduced to Fortified so you can't use it to destroy a position entirely, and the defending Power can easily push it back up to Stronghold (without the carrier) the next week.
- Exploration data over-effectiveness: was shown on stream anyway because someone used it to try out the Concord Cannon.
- Rares being too good: probably would have been picked up, but it'll take a bit more effort on refining approaches before we can tell how high some of the more complicated/combined ways of getting merits cap out
- Data Port exploit: might have been spotted, might not have, depends on the size of the testing group as to whether someone accidentally does it and notices. Took about a week of live play for someone to notice so the odds of a smaller group finding it are low even if they're deliberately looking for loopholes.
- 1t trade exploit: was anyone in the test group old enough to remember it working in the pre-3.3 BGS? It's taken almost two weeks to notice in live.
- mixed activity rates: very easy to pick up on just by doing each activity in isolation that A is faster than B, but a lot of the interest in PP2 is whether you can effectively
combine multiple activities to get A+B merit rates ... and then also "passive" merit activities (scans, normal trade, mining, bounty hunting) can afford to be less efficient as merit sources because players are already getting other things from those activities too, and they're likely to be done by pledged-but-not-actively-Powerplaying commanders. That was always going to need a lot of live balancing anyway.
(Like: on paper and in isolation the various Odyssey activities all look pretty poor; even without the data exploit doing all of them at once is actually pretty competitive for non-SC undermining)
- the various documentation issues: yeah, they'd have come up straight away but aren't a balancing problem as such
- anything big-picture: no chance that a small group of testers can actually duplicate whatever structural mess Powerplay gets itself into through collective actions in about 3-6 months time
[1] How much of the effectiveness of rares is because non-Mahon Powerplayers can just rock up to the Old Worlds cluster, pick up their rare goods, and go again, without having to worry about the Mahon forces in the system? Or grab some Harma Rum right out from under Archon Delaine's nose? If you had to fight your way in a bit, and then possibly fight your way through the systems in-between the rare source and your target, that might slow it down a bit!
[2] It's exactly the sort of offence-over-defence balance point which a lot of players absolutely hate because loss-aversion is the name of the game ("Dangerous" refers to how angry players get if they might lose something, right?). But that doesn't mean it's
wrong - Stronghold Carriers are really useful, there probably should be some sort of requirement to maintain them or lose them, it's self-limiting because of how SCs are generated, your HQ carrier is immune, etc.
Similarly the data port exploit absolutely needs shutting down hard and fast ... but I feel a lot of the anger over it is specifically because it's a fast undermining technique and Frontier removing a fast fortifying technique (like Orrerian Vicious Brew) instead mostly gets "why has Frontier nerfed this

" reactions.