NopCommerce has spent the last 5-10 years pouring most of its development resources into keeping up with the ever-changing .net ecosystem. You can imagine what a code base is like when the framework it's built upon is being swapped out every one to 2 years.
Its third-party ecosystem is sorely lacking.
The schema it uses for product attribute combinations is brittle and poorly designed, making fine-grained discounting and reporting almost impossible, among other things.
Redirection style payment methods will leave you with an empty shopping cart if you return to the site before completing payment (eg, to verify your cart's contents).
I've reported easily reproducible bugs and the devs simply refuse to fix them (such as double discounting for associated products), seemingly due to them not taking the issue seriously or failing to understand the issue.
tldr - NopCommerce sucks.
Pro - Slick UI - Solid backend to manage products - Good plugin support with other e-commerce giant like Amazon, eBay - Good community support - Flexible store customization with good range of theme - Easy to manage even for even non-IT person(Taking about backend process like product listing, shipment generation etc...)