Top vs side navigation: Which one is better for your product?

This can change depending on the device and font sizes, but generally, you would be able to show at least double the number of navigation links in the side nav above the fold (not mentioning the scroll). 

Side navigation works better when your information architecture has many top-level items that cannot be logically grouped into several buckets. This also means it's much easier to add more items as your product grows.

The scalability of the side navigation makes it the only choice when users can configure their own navigation. 

Variable menus are quite common, be it channels in Slack, folders in Outlook, or hierarchical menus in Drive or Confluence.

The left navigation with a drawer-style submenu has issues with diagonal cursor movement. In addition regardless of the number of items in the submenu, with side navigation most likely you will need to block another vertical pane on the screen.

This edge that top navigation bars have with hover-activated menus, is even more apparent with mega menus that are predominant on e-commerce and other big websites. 

They are an excellent design choice for accommodating a large number of options or for revealing lower-level site pages at a glance. This layout also leaving more space for product display and ads.

• For top navigation with not too many navigational links, you have the benefit of keeping all of those links visible on tablet portrait views. 
• Side menu most likely will be hidden in the hamburger menu on tablet portrait, but has the benefit of keeping the navigation view consistent across all screen sizes.

• Top navigation: Uses little space and takes a prominent position on the page. Works well when there are not too many navigation items. Use primary top navigation for small, medium, and large websites, e-commerce, and web applications that don’t have a hierarchical structure.
• Side navigation: Supports products with a large number of navigation links, easily scalable and configurable. Use primary side nav for complex applications and websites, admin apps, desktop apps, and file/content management products where users can customize their navigation.