The developer tools are getting better but are not as good as Chrome. For instance, you can see Ajax requests being made and responses but you cannot see the headers or info that was sent in that request.
As Vivaldi is built over Chromium it has the same bad RAM performance of Google Chrome or any other Chromium browser. As the Vivaldi UI is also technically a webpage it uses slightly more RAM, around 40MB more.
The high performance and speed come at a cost of an abnormally large RAM usage when it comes to Chrome. This isn't a problem with machines with lots of RAM (4GB+) but it's very noticeable with computers that have less RAM than that and can slow it down a lot depending on the number of open...
When browsing around, Opera is quite efficient. But when loads of tabs open, it feels like Opera is putting a great effort to keep all those tabs like awake or active instead of just the top most one. This can heat up a laptop and drain its battery.