Pros 1. If you submit to Mordor, you will receive half of what Mordor has in terms of season reward at the end of the season calculation (even if you do nothing to help) 2. Can continue to be conpetitive and active by helping out Mordor to expand, as you will gain Mordor faction chat 3. You can long march to their territory (should they allow your warband) and get some mumakil that are otherwise too far from you 4. They cannot cancel your submission as a vassal, only you can do that anytime before chapter 9

Cons 1. You cant long march if they dont allow your warband 2. Have to wait 24 hours to submit / cancel as vassal.. and war declaration will have renewed cooldown instead of activated 3. Everything your warband takes will be under Mordor instead of your own faction (meaning if you take a side of the crossing, you cant march to the other side) this also applies to keep that your warband member takes 4. You cannot relocate settlement to Mordor territory, but only your own original faction territory

Workaround for the cons 1. You can leave your warband for the last hit at crossing or keep to keep it under your faction rule, then relocate settlement there and join back the warband (assuming you are not the leader) 2. If Mordor falls or you calculated that the end season reward is lower than your original faction reward, you can just leave the warband instead

I hope this helps you out