Foreign stocks have two extra sources of risk attached to them; exchange rate and political.
Exchange rate risk is obvious; if I buy a stock in a foreign currency and there is a currency movement that makes that investment worth less I lose money no matter what the stock does. This can be offset using exchange rate swaps. (This is ceteris paribus, of course; changes in exchange rate can give a comparative advantage to international and exporting companies that will improve the fundamentals and so increase the price of the stock relative to a local firm. The economics of the firms in particular are not explored in this answer as it would get too complicated and long if I did.)
Political risk relates not only to the problems surrounding international politics such as a country deciding that foreign nationals may no longer own shares in their national industries or deciding to seize foreign nationals' assets as happens in some areas. Your home country may also decide to apply sanctions to the country in which you are invested thus making it impossible to get your money back even though the foreign country will allow you to redeem them or sell. Diplomatic relations and trade agreements tend to be difficult. There are further problems in lack of understanding of foreign countries' laws, tax code, customs etc. relating to investments and the necessity to find legal representation in a country you may never have visited if there are issues.
There is also a hidden risk in that, as an individual investor, you are not likely to be reading the local financial news for that country regularly enough to spot company specific issues arising. By the time these issues get into international media its far too late as all of the local investors have sold out of their positions already.
The risks are probably no different if you have the time to monitor international relations and the foreign country's news, and have FX swaps in place to counteract FX risk as the funds and investment banks do but as an individual investor the time required is not feasible.