A well known bank in my area mentions you can link external bank accounts and superannuation funds to your one bank account so you can conveniently see all your funds and net worth in one place, but are there any disadvantages to allowing your bank access to seeing your other external accounts and funds?
Any linking (even for ACH transfers) exposes you to more risk from when/if a thief (anonymous hacker or family/friend) breaks into your bank and starts taking money.
As far as direct linking... now the bank sees more of your personal information, and buried in the Terms and Conditions very well might be the right for the bank to use that information for purposes not in your long-term best interest.
Personally, I'd never do this. It's also why I don't use Mint or other account aggregators.