I see big clans forfeit warbeasts or other small epic battles all the time. Saying that it isn't worth the time/troops is a better answer IMO because not every big clan that does a forfeit is in an osw.
EBs like ambush are run so hansels can pin attack troops and not get farmed. Since attack builds put spies and troops on enemy, and hansels do little EB damage with attack troops, EBs are left unfinished and forfeited at the pot stage, because why waste pots on an EB you can't complete? Ambush is done by clans with low bfa hansels because they can hit without wasting pots. High bfa hansel clans often choose city of the dead because it has a long hitting stage before you get the pots stage so you don't need to restart the EB so often. Hope this answers your question.
Epic battles are often used as pin cushions by hansels or to fund strips and repot but mainly as pin so im assuming when it gets to a phase they cannot use troops they forfeit the EB?
There in a off system war(osw) with another clan so there only using troops on the battle while spies go to other people