I have BAEC and AAdvantage accounts (and a CX Marco Polo account for that matter - I only mention this as these are the OW FF programs I'm familiar with) , when possible I ALWAYS bank my miles on OW airlines in my AA account, for various different reasons. Here's what I dislike about BAEC:-

BA charges outrageous YQ prices on award bookings.

BA has three different zones in Europe and mileage requirements differ accordingly.

BAEC think nothing of putting the customer on hold for a looooong looooony time at least when I've called them from Canada, not sure if it's any better in the UK.

Overall, I need to redeem more BA miles than AA miles for the same award.

To be honest, the only think I can think of that I like about the BA program is that you can have a household account (not 100% sure if the rules change depending on where you reside).

If your brother accumulates a lot miles from credit cards, obviously it doesn't make any sense to have a card that would charge foreign transaction fees.