Noticed this only for CS 767 but not with CS 757 :
When flying LH @x2 x4 or x8 speed the flight crash around mid-point - I think it seems to be linked with fuel actions to be performed (switch off pumps) when the 2 first rear tanks near empty. Even if other tanks are filled the engines quit and plane crash !!! (all the while still in 2x, 4x or 8x accelleration)

Found also : fuel rates are off when using acceleration - I begin a flight where estimated fuel left is near 2T and the real figure would be like 20T left ! Like the FF is not multiplied if you wish...