If there are three Merrill accounts and one of them is left off the return, IRS will not match the totals shown on the return to the individual transactions they see on their CP-2000 worksheet.  They will conclude that there is zero basis for all the unreported sales.  I have seen that happen.  Of course, they would probably screw it up even if they had all the individual sales.  

Listing all the transactions was a pain back in 2013-2015, when most of them were uncovered.  Now, most of them are covered, and all we have to deal with are the wash sales.  IRS is still requiring data that was useful back in the transition years, but useless now.