negative equity/tangible book value

n many of Buffett's letters, he includes the increase in book value per share of Berkshire as if it is a metric he watches closely. I've noticed that many large companies have a negative tangible book value and most of their book value is goodwill or other intangibles. Similarly, the Equity (assets minus liabilites) is also negative for many companies. Since equity is essentially the net worth of the company, I would think you would want this to go up because this would be your net worth if you owned the whole company. However, if you look at stocks like Mastercard, the tangible book value per share is negative yet they have huge cash flow per share and the stock has gone almost straight up for years. At what point is a negative tangible book value/equity a problem? Using Costco for comparison, it has a high return on capital and the tangible book value/equity is growing as is the cash flow per share. Thank you all.