You'd better avoid trading exotic pairs if don't have rich trading experience behind your back and if you don't know anything about the economy of the countries which these currencies represent.
The major obstacle, from my perspective, is low liquidity. What does it mean for traders? Well, you'll have lots of slippages and high occasional volatility. Surely, volatility is good for traders because it is actually the only way he/she can make money on trading forex. However, if the volatility takes place and the trader wasn't prepared for it, he is sure to lose lots of money, especially if he doesn't follow the principles of money and risk management. So, nobody prohibits you trading Brazilian reals or Turkish liras, but you need to be prepared to all the risks associated with such currencies.
The major obstacle, from my perspective, is low liquidity. What does it mean for traders? Well, you'll have lots of slippages and high occasional volatility. Surely, volatility is good for traders because it is actually the only way he/she can make money on trading forex. However, if the volatility takes place and the trader wasn't prepared for it, he is sure to lose lots of money, especially if he doesn't follow the principles of money and risk management. So, nobody prohibits you trading Brazilian reals or Turkish liras, but you need to be prepared to all the risks associated with such currencies.