To be fair, most people don't know much about everything. Anyone that's survived survived a trip into tainted land is a 'veteran', but delves are often as bewildering and disorienting as Willowbrook. Most mercs pick up tricks with how to survive and how to escape, while others find records or codices which teach them unique things like how to fight certain enemies or cure certain curses. The range of skill, methodology and experience between veteran mercs varies so wildly there's no real baseline of comparison between them individually. It's largely luck and idiocy, the smart ones that survive usually wouldn't roll the dice a second time.