The Invesco QQQ Trust tracks the Nasdaq 100 Index, which is simple in construction but complex to consider within a portfolio.The Invesco QQQ Trust (QQQ 0.14%) is highly successful at what it aims to achieve. But that's also a potential problem for investors.This relatively simple exchange-traded fund (ETF) needs to be carefully considered before you make it a core holding in your portfolio. Here are the details that will likely be most important for you to consider.What does the Invesco QQQ Trust do?The In ...