(You mean search engine, right? I can’t find a browser they’ve released…)
I definitely like the concept – and that may be worth using it for anyway! – but if you’re focused on privacy, it’s iffy. The page says that they don’t sell your data and have no third-party trackers (though notice they don’t say there’s no tracking going on at all, like DuckDuckGo does), so theoretically it’s better than just using Google, but it’s not open-source so there’s no reliable way to confirm that
A rule of thumb is never put too much trust into closed software. Since the code isn’t just out there for anyone to look through, at best your assurance is the handful of people who were given permission to look through it and tell people it’s secure. But there’s no guarantee that they weren’t paid off, or didn’t miss something, or didn’t look the other way from the backdoor because someone assured them it’d “only be used to stop criminals” or something. And that’s if they’ve even had security experts look through it – sometimes it’s just the word of a programmer
privacytools.io is a good resource for software that will respect your privacy. Their page on search engines doesn’t mention Ecosia, sadly, but has several good alternatives. I use DDG personally, and it also comes default on Tor so you know there’s a level of trust in it