Also, because Berkshire is a conglomerate, businesses which generate large amounts of cash (e.g., insurance float, or businesses that would pay dividends if they were a stand-alone companies) can be used to fund capital-intensive businesses (railroads, energy companies, etc.) without paying taxes to move the money from business A to business B, because they're all owned by the same legal entity. Don't underestimate the power of this effect. It is not possible to achieve with an index fund.
Berkshire is in the SP500, but so are Apple, Coca Cola, Amex, BoA, etc. which are Berkshire largest investments. So if Coca Cola has an idiosyncratic hit, then you get hit twice by it: first in your KO holdings, then in your BRK.A holdings.
At the extreme, if there is a company that then invests in Berkshire and so on, you could end up overweighting a lot certain firms. Also, isn't there double counting in terms of the stock market cap?
I think that is neglibile, depending on how big you define a hit. If Burger King gets hit, that will probably effect Coca Cola as the largest supplier (just saying it like that, don't know if they really have that kind of business with each other), too.
There's lots of dependency in the modern world, I don't think it really matters.
edit: If Coca Cola does well, you profit twice. Maybe see it that way... ;-)
"Float adjustment excludes shares that are closely held by control groups, other publicly traded companies, government agencies, or other long-term strategic shareholders."
[1] https://www.spglobal.com/spdji/en/documents/index-policies/m...