Out of all the names presented so far, the one that truly sticks out as a sore thumb is Alessandra Faccinetti, for no other reason than the fact that at both houses she inherited she was given anything but a fair shot at success. Both of her stints lasted only one fashion year (a s/s and a f/w season). Talent certainly isn't an issue, and even though there are mixed feelings about her work at Valentino the truth is that she had a firm grasp on both of the legacies she had to work with.
With her I think it was a matter of not getting what both Gucci Group and Marzotto/Permira bargained for. Obviously it's just speculation on my part, but given both the circumstances under which Tom Ford left Gucci and Frida Giannini filling the role of creative director after her two co-directors left after brief stints at the helm, it seems like Gucci Group wanted much more control over the Gucci label, everything from the products (all of those pre-season logo bag editions, the general mainstreaming and pairing down of the clothes that end up in stores) to the image of the house (the constant pushing of Gucci's heritage, the sterilization of it's hedonistic former image). I'm guessing there was some internal conflict over creative control that was never disclosed, because two full seasons worth of clothing sales doesn't seem like enough time to judge whether or not a designer's product sells well. I can't believe that sales were what got her fired, especially when RTW isn't the big money maker to begin with. I'm guessing it would have been easier for Gucci to exert control over someone who is out of their element, such as an accessory designer working as a creative director of a fashion house.
As for her stint at Valentino, I just think she upset people by having her own point of view and not sticking with the exact same course that the house had been on. Clearly that was what upset the founder himself, and all of a sudden when the news broke that she was being let go the company was agreeing with him.
Obviously this is just something I concocted, and isn't actually based on anything.
Don't forget Alexander McQueen, John Galliano and Julien Mcdonald at Givenchy
Those were very different situations in that none of those designers were unceremoniously ousted from the house. Galliano left after two seasons because he was offered Dior and made the choice to leave. McQueen's contract wasn't renewed after three and a half years due to uneven reviews and sales, the bitter working relationship between him and LVMH, and his choice to sell a stake in his label to Gucci Group while he was still under contract at Givenchy (I'm guessing that last one was the straw that broke the camel's back). Even still, they just let his contract run out, he wasn't actually fired. Macdonald as well remained for the duration of his contract (3 years, I think), even though his time at the house was hardly a success.