1. I'm currently using coinbase as my main crypto exchange. But I keep hearing from a lot of people who have been trading/mining crypto for years say that they don't use coinbase anymore. Why is that?
The reason is that coinbase is much of a centralized exchange and they make US SEC rules their top priority. They can freeze your coins, ask you to go through KYC anytime without a genuine reason and coinbase is also not good for trading cryptocurrency.
Also what exchange is the most popular to use right now? Is it still Coinbase?
No, it depends on your geographic location but coinbase is the popular exchange in the US but Binance.us and Kraken is a better alternative.
Is it normal to make several purchases on coinbase (using your checking account) as in purchasing portions of several different types of crypto, for those transactions to be pending for a little while before your available balance reflects the amount that you actually purchased? How long does the pending phase usually take on coinbase, if your checking account is in the United States?
Generally in 2-3 business days but it hard to tell now how long the transaction will take since coinbase is currently experiencing an increase in traffic and it will take more time than usual before the transaction reflect.
Read this
link for more insigght.
I've heard mentions in forums and youtube that people who trade prefer to store their crypto in wallets outside of an exchange for security reasons. If I'm understanding that right, that instead of storing their crypto in wallets that you can make on your coinbase account, they store it in a paper wallet or something. Am I understanding that right? And if so, what security risk am I taking by storying the crypto that I've invested in, in wallets that I created in my coinbase account?
Yes, people are advised not to keep huge coins on exchange because of some previous hack issues that have happened to some exchange like Bittrex, Bithumb, etc and it may also be from their end just to steal users funds that why people are advised to keep only the coin they can afford to lose on exchange. But in your case, I will advise you to get a hardware wallet from ledger recommended retailers because there some certain steps required to get a secure paper wallet.
With that been said, you can use an exchange like Binance or coinbase to hold some certain coin but getting a hard wallet that you can save in a safe deposit box is good for holding your long-term investment than keeping it on an exchange, the exchange owner understands this that why they introduced Roi (% interest) for the user that keeps his holding on their exchange.