I've got 10% of my portfolio in Bitcoin and Ethereum (50% EACH).
Each time either of them grows 20%, I take out half the profits and buy proper JSE stocks with the profits. That way, I've taken out several times my initial investment, so if it plummets in the long run, I've still scored massive on its growth. I think the risk is only if you get greedy and leave everything there that you may get hurt in the long run.
Just come up with a good risk control strategy. My strategy of waiting for 20% growth and then taking half the profit and using it to buy on JSE has worked wonders. I've already made back several times my initial investment, so if stocks collapse to zero, I'm still okay.
In other words, I'm using bitcoin and ethereum short-term as my JSE stocks funds generator...