Shorting great companies with high growth, because of high valuation, is a noob error. Instead, focus on companies whose business model, solvency, or earnings are going to deteriorate significantly. Stick to stocks where the earnings go negative, where the company can't pay its debt, or where the business model has become obsolete; not companies which are going to be much bigger and more successful in 3 or 5 years' time.
If the valuation is truly insane, rather than merely expensive, then you can make money shorting great companies. But it's an uphill struggle. Any glamour stock can come out with an earnings beat, new product, new deal, which can pop the stock 30-50% in a few months or even weeks, and your short is a dismal failure - even if you eventually are right, you make 50% after suffering a 50% drawdown, a marginal result to say the least. And these kind of stocks can just go up 50 or even 100% on pure air, if sentiment improves and the bull market continues to pump up valuations. Finally, some stocks like this will just beat earnings estimates quarter after quarter, and grow into their valuations far more rapidly than expected, and that's where you can lose huge amounts of money. There are too many factors against to be worthwhile.
Given that even in the midst of a raging bull market, there is always at least 5-10 stocks whose businesses are bad and getting worse, whose earnings are in the toilet, whose management is incompetent or criminal, why the hell make a marginal, risky trade on a great company and a market darling with momentum players up the ying yang? Hell, every single year there are at least a dozen stocks that are *outright frauds* worth literally $0 per share, that will eventually fall 100%. It is inferior trading to short a stock like NFLX when you can short a total fraud, or an impending bankruptcy.
