Its very risky because they might be delisted in future for low volume trading on exchange which can make you loose your investment.
You are talking about coins that are far under top-400 but those that are under top-100 have great trading volumes. SYScoin, Zcoin, Dragonchain, Storj, Civic, iExec, SONM and others - they are listed on good exchanges with good volumes, so it is not so risky to invest in such coins from top100+
If we talk about the risk, everything is risk. We must take the risk to try the opportunity to get something as reward.
But top 100 is not very risky, because its better popular and also have better volume. But sometime it can be down below 100 because loss popularity.