While it is true that the assets that Buffett has gotten into are diverse, he feels that diversification of a portfolio is not something that is always going to work out right. The problem with diversification is that it might involve trying to find businesses in random fields. The key is to focus on appropriate cash flows and beneficial investments even if this means finding options that are not always going to be very diverse in nature.
One interesting opinion that Buffett has is that getting in on initial public offerings is never a good idea. This is due to how it might be too unpredictable to see how well an IPO is going to work. Buffett prefers to go along with investments that have proven themselves to be of some use in recent years.
Buffett also tries to avoid investing in online and social media companies. Although he does respect these companies, his opinion on investing in these companies is that it is too difficult for people to figure out whether or not stocks in this field will have decent values in the future. The lack of predictability and the lack of knowing how to forecast this point has been a part of the problem according to Buffett.