I haven't not had any problems with my BOA boots getting loose. I am beating on them in ways that most people don't as I mostly splitboard to get my turns. Which is also the main reason I use the BOA focus system so that I can loosen the uppers for the skin up and tighten them up on the down. Super quick and easy.
That said, laces are tried and true. For some styles of riding they are probably the best option. Also, they are very easy to replace when you break a lace. Not that BOA is hard to replace, but laces are still easier.