The phrase "unassailable fact" means a statement or information that cannot be challenged, proven wrong, or disputed, as it is based on solid evidence or is widely accepted as true. Full definition
Whilst Wenger's obvious crimes and mismanagement follies includes a few unassailable facts; he bought him, kept him, loaned him and brought him back to cover an injury (it would appear the greatest crime was having the mere thought to possibly release him). (justarsenal.com)
In the coaching world, there are a lot of assumptions that have hardened into unassailable facts. (blog.trainheroic.com)
How do you suppose this type of information reflects on those rookies who already have an uphill battle to fight to land a client when they're met with a the potential client who, because they have your erroneous chart, thinks they're armed to the teeth with unassailable facts because someone of your stature published it? (remonline.com)