What I mean by that is that the game has incredible amounts of inertia, much like Monopoly which among board game enthusiasts is notorious for its completely awful game design.
If things are going well. They continue to go well and the mechanics make things better.
When and if they start to go bad it quickly devolves into chaos and an unwinnable scenario. With no catch up mechanics to help. Here are some things wrong:
The 10% increase in resources when everyone is happy just makes winners keep winning. When everyone is unhappy however you lose that bonus and they are unhappy *because* you don't have resources. So losers continue to lose.
The radio station is a trap, it shouldn't be used except to raise happiness levels. Babies can give people at a faster throughput and the radio station doesn't level its inhabitants. It also draws dwellers away from producing resources.
Rushing rooms is untennable in the situation where you need to most. When everyone is sitting at half radiation and the water is in the red you would want to rush but the consequences of doing so could wipe out the vault since everyone is low on health. The only reason to rush when everything is ok would be to get quick caps and xp, and when everyone is high on health you can rush without fear of incidents because they can be handled easily.
Much of the success is determined by the first 3-5 lunch box pulls you receive for free. Which I suspect may be gamed from the start...But I won't get into that on this initial post. But suffice to say if you receive an excellent weapon or character or both you will be able to succeed much better than if you get caps from the initial pulls. The weapon effectiveness will carry you through attacks and high stat characters will pull their weight in the proper rooms. Whereas caps will only get you resources to build or upgrade rooms you can't staff and upgrading will make attacks stronger and you won't have the weapons to rebuff them. This might be the biggest problem. Getting caps from lunch boxes and using them is a death sentence.
High level dwellers generate tons of revenue and help to mitigate attacks. When and if they die it is a huge expenditure to bring them back which quickly drains caps. They are all important to revive though because of their revenue generation. IMO they are too expensive to revive, and sometimes unclickable when running around in an emergency. Reviving a level 1 character is 100 caps which you won't make back on that chracter until they reach their 14th level... That's waaaaaay too much! I saw a mid teens I think 15th level character revive for around 400 caps. That investment won't return until level 32. Why have a revive mechanic if it is too expensive to use?
Lastly there is a lack of a catch up mechanic. When things go bad, which they inevitably do there is seemingly no chance to recover. A lunch box every 7 days doesn't get anyone off their feet again. Rush chances only get worse in desperate times. And high level dweller deaths remove any caps from the equation.
The game is basically the player trying to prevent a landslide from occuring where everything is just peachy and then suddenly awful. It shifts from extreme highs to extreme lows very quickly.
If this was the aim to help explain why so many vaults end up like they do... Then great! But that doesn't sound like a fun game.