Yes, you risk buying at inflated prices let's say. The common idiom is "time in the market beats timing the market", meaning yeah, just get your money in if you have it. Here's one analysis [1] that claims since 1950 in every 10-year period, lump-sum would have been better 75% of the time. But there are some caveats.

First, 25% of the time is a lot of the time. It may be possible to recognize an overheated market, by looking at historical earnings multiple averages for example. Second, they are using a "total market" index, even broader than the S&P 500. I don't know that many people who buy a total market index. Usually it's the S&P 500, or even a sector focus. The more specific you get, the more these results are likely to break down I would bet.

We focus on DCA for people who are working for a living and investing out of income, which is obviously a different cohort than Fred Wilson, the VC. So lump-sum investing doesn't really factor into it.

[1] https://www.northwesternmutual.com/life-and-money/is-dollar-...