I think "Fancy Balance Issues" is becoming an overused term, a meaningless buzzword. FBI is something to watch out for, but "situationally useful" or "niche" does not equate to having FBI. I think FBI should refer to cards that are usually useless, even detrimental, but in perfect situations become gamebreakingly powerful.
Does Astrolabe fall under this category? We'll need to wait for playtesting, but I speculate that it will not. Consider:
1. When it is weak, it still isn't useless. Maybe you have a simple BM-X strategy (Smithy, perhaps) that is fine at getting $8 or more. Adding in an Astrolabe isn't great, but sometimes it will pay off.
2. When it is strong, it is not gamebreaking. Now, this point is contentious -- some people think it is gamebreaking. But consider the situation when it is arguably at its strongest -- powerful draw engines. Now suppose in a 2p game, both players pursue the same engine except A picks up Astrolabe while B just starts greening earlier. When they start greening, A is able to hide green away. But B has a head start, and that may mean that B will win the Province split. Maybe Astrolabe, being a Copper, prevents A from getting double Province while B manages. A gets to keep dead cards out of his deck, but is that a big deal? If both their engines are drawing their deck anyway, will an extra dead card or two cause them to choke all that much? I think that it usually won't. If both players are playing strong engines, the Provinces will drain quickly before there will be much choking.
That isn't to say that Astrolabe can't make a difference. I think that it often will. But it won't be facerolling must-buy powerful -- it'll be another component that you have to fit into your overall plan. When do I pick up Astrolabe? Does it allow me to start greening earlier, with an engine that isn't as powerful? Will a stronger engine without Astrolabe be able to catch up?
Now compare it to Talisman and Quarry, which are also alt Coppers.
Talisman: Good when the key cards are cheap, bad when they are not. You'll often want one to help win a split of, say, Fool's Gold or Caravan. On a board with no power $5 cards, this could be the dominant strategy and if you don't follow it you might lose... right? Maybe not though. But I feel it works out similarly to Astrolabe (though with different "powerful" situations, of course).
Quarry: I think it's more pronounced here. Quarry is rather useless when the dominant strategy is BM-X. But when there is a strong engine to build? Man, Quarry is so good. If you don't pick it up, you will probably lose the BV-Wharf split, and all the GMs, and then you've got no chance. But does Quarry suffer from FBI? I think it's still fine.
I think people are overestimating its effect on engines. If Astrolabe isn't on the board, that engine is probably still the best strategy. There may be rare cases where Astrolabe makes an engine viable when it is otherwise too weak, but I think that's niche and interesting. With the strong engine, Astrolabe becomes another engine component you have to factor into the build.