There will always be overvaluations and undervaluations of companies relative to how they'll perform in the future. It's notoriously difficult to predict. In my opinion, however, while there may be some overvaluations now, we're not even in the same
universe as we were in 1999/2000.
Back then you could take an MBA with a powerpoint, put the word "Internet" on it, and they could conceivably IPO for hundreds of millions with nothing more than that. All of these companies valued at $100m, $1b, $10b with no more than a promise of "one day we'll sell x online and make a lot of money." A couple companies full of genius technologists (Amazon, Paypal) made it through the rubble, but for the most part it was all smoke, mirrors and charades.
Contrast that to today. Certainly there are overvalued companies; there always are, but there are also companies that are proving to be worth the hype, and severely undervalued. The Internet is 20x as large, there's real revenue coming in, real profits, and the scale that companies can hit when they land on something solid is incredible.
People were crying Bubble when Facebook was valued at $30B because it wasn't making much money. Now it's bringing in billions easily and still growing http://www.marketwatch.com/story/facebook-profit-tops-1-bill..., now valued at $400B+. So some companies were undervalued, even when everyone was crying "bubble."
As an asset class today tech is tiny. The valuations of every tech "unicorn" combined today would be less than Microsoft. Would you rather own every single tech startup that raised venture capital in the past couple years or Microsoft? You could have that discussion, but you wouldn't be crazy to take all of the VC-raising startups. And today the total amount of venture capital raised is a rounding error relative to the rest of the economy. (There's also a discussion around how all asset classes are highly valued today and even bonds are returning negative yields, but we'll save that for another day).
Tesla is certainly expensive relative to where it's at today, yet I still put in my entire net worth at $180 http://www.marketwatch.com/story/elon-musk-to-the-guy-who-in... (this is not a move that would be recommended by your financial advisor; my net worth is something I could replace quickly enough that it's not a big deal). Tesla is, in my opinion, building a fortress. It's combining the technology needed to incredible electric cars at large scale with the biggest battery factories in the world, solar-powered roofing, a nationwide charging network, and just some incredible innovations. We've seen Tesla do the impossible an innumerable amount of times in recent years.
So the price of Tesla right now is very frothy, but I still think in 10 years I'd rather own 100% of Tesla than 100% of Ford. If you disagree, then Tesla is overvalued, but no one truly knows who will be right. Such is life as a growth stock.