PAYPAL DUPLICATE TRANSACTIONS: I couldn't figure out why I had up to three transactions in Quickbooks for every one PayPal debit card transaction. Frequently, there would be two debits and one credit in Quickbooks. It turns out that there is no workaround to prevent this from happening, even in the new software I'm using, but the new software clearly showed me what was happening with my account, which made finding a solution easy. (I'm currently in the process of switching accounting software.)
MISTAKES: Quickbooks also does not allow you to easily undo mistakes. Sometimes, you simply cannot. :-/
MOBILE APP: I get an error every time I try to upload a picture of a receipt using the mobile app. This is why I dinged "reliability."
RESOLUTION: With the PayPal issue, Quickbooks required that I unlink PayPal and connect it a certain way. This deleted all of my work with my previous transactions, such as categorizing and uploading receipts.
NOT FOR BEGINNERS: As someone who catches on quickly, QuickBooks was NOT intuitive (unlike the new software I'm switching to). Even after figuring out my PayPal issue, QB seems like too much of a pain to continue with.