While USB-C is steadily gaining in popularity, it's not ubiquitous yet — and that makes it harder to choose the Pixel 3.
USB-C is generally considered superior to other connection methods, including Apple's Lightning port, because it's sort of a jack-of-all-trades: Not only can a USB-C cable charge supported smartphones, tablets, and laptops alike, but it's also a standard connector for a new breed of accessories like monitors and headphones.
Smartphone makers are beginning to adopt USB-C, but it's been a slow process, and it can make things tricky if you have a phone with a USB-C port. For example: say you're at a friend's house for a party and you need to charge your phone. It's almost guaranteed that among a group of people, someone will have an iPhone charger. But try asking for a USB-C charger, and you're likely to be met with blank stares.
This can be an issue in public spaces, too. Thankfully, in the case of the Pixel 3, it now supports wireless charging, and you can increasingly find supported charging pads in spaces like airports and Starbucks.
It's hard to find a USB-C port, too
There's one more issue that I've found with USB-C charging: in the case of Apple's Lightning charging cables, the end that connects to the charging brick is the old-school USB-A connector. At this point, USB-A is everywhere, like most laptops and even your car; if you need to charge your phone in a hurry, you have options.
But the cable included with the Pixel 3 is USB-C on both ends, which means if you don't have access to a wall outlet connector, you need to find a USB-C port to plug the cable into.
For me personally, this means I can't charge the Pixel 3 using my MacBook Air, or using the USB-A ports that are built into my desk at work. It's a super-specific problem, to be sure, but a problem for me nonetheless.