I thought it was about whether the PC's were aware of those factors... not whether they impacted or didn't impact the chance for success...
Answering for myself, hidden backstory can invalidate a PC concept from the very start of the game, but the player him or herself may not discover that for ages. Mainly I'm talking dealbreakers here, situations the player will find untenable and will lead to characters being killed off, retiring or players leaving. The referee may or may not notice this. If s/he doesn't play proceeds until the big reveal happens, and the consequences kick in. If they do notice, they need to decide whether they prioritise player enjoyment or keeping secrets more. Early on player concepts can be nudged to more appropriate or less doomed directions without letting on exactly why, though players may guess right.
Lots of little obstacles do accumulate, and if the pro-activity of a player runs against the grain of a hidden backstory, they can inadvertently be locked into a cycle of failure, where the referee is reluctant to explain why so many things the player does seemed to be doomed to failure or turn out irrelevant. Even when their is no single dealbreaker, a consistent string of failures of pro-activity will sap the morale of most players (unless they enjoy tragedies). Too much dangerous hidden backstory turns the game into a game of minesweeper, where the player is forced to make blind moves and any move could be his or her last.
If and when the player finds out they can be very cross their playing time and character investment has been wasted by the referee (as they see it), who typically figures out before the player concerned that they will be displeased as to the turn of events, but may be reluctant to rock the boat and hope against hope that everything will work out right.
Hidden backstory is always a risk, a double risk as it's difficult to get prior feedback or buy-in from the players without blowing the secret. Tastes differ, and I've seen a high proportion of hidden backstory and plot twists blow up in the referee's face as a consequence.
For me the nub of the issue is whether a referee prioritises player proactivity or hidden backstory and secrets.