Investing that large of a percentage of one's portfolio in any single company is extremely risky; even if it is a large, publicly traded giant like Microsoft or Amazon. In order for the investment to be successful, not only does the company have to do well, but their sector has to do well, their country's economy has to do well, and stocks as an asset class have to do well.

If you don't have hedges to insure against those risks, you are still essentially gambling with your principal. Even a Series A/B Ycombinator company probably has a double-digit percentage failure rate.