Why in the world would I recommend a single stock? For one, Berkshire is a "financial fortress" with virtually no debt, $100B+ in cash, and about a collection of dozens of wholly owned subsidiaries: GEICO, See's Candy, BH Energy, BNSF Railway, etc. Berkshire's so conservative that, although Buffett and Munger have acknowledged leverage could boost their returns, they refuse to take on obligations that would even create a 1 out of 100 year risk of failure. More importantly, I believe it's cheap at current prices (a little under $200 for each class B share). When you buy Berkshire, which has a market cap of ~$500B, you're getting $175B worth of stock $125B in cash, and the all the subsidiaries for $200B. And I'd bet the intrinsic value of those subsidiaries exceeds $200B by a substantial factor.
Risks: (1) The biggest problem is that Berkshire is already quite big. That makes it difficult to compound capital at large rates. But compared to the S&P 500 or the even-bigger tech companies, I'd rather be buying Berkshire, at least at today's prices. (2) Buffett and Munger's advanced age. This to me is not as huge of a problem as people make it out to be. Todd Combs (GEICO), Ted Weschler (investment manager), Greg Abel (non-insurance ops), and Ajit Jain (insurance ops) are extraordinarily talented... to say nothing about the leadership in the subsidiaries. And Berkshire's built such that it would be a great business even without these top-notch managers. (3) Time. This isn't a problem if you're happy to hold Berkshire for 20+ years. But markets are emotional beasts and stock prices can decline rapidly and unpredictably. Berkshire isn't a bond nor does it pay a dividend. So don't buy Berkshire if there's a risk that, 5 years from now, you'll need to sell or if you're the kind of person who can't deal emotionally with large short-term declines in stock prices.
Disclaimer: NOT investment advice. Just my personal opinion. I've been buying up more and more Berkshire in the past few weeks.