Fanatical is as trustworthy as Greenmangaming, Gamesplanet, Humble, Gamebillet, indiegala among others, and according to TrustPilot it's got a rating of 4,6 / 5 with 75.000+ reviews:
https://www.trustpilot.com/review/www.fanatical.com

I always use sites like gg.deals to see where the best price is and to see which one is trustworthy or not, and I've used many of them (can't recall if I I've ever used Fanatical specifically, though...).

As long as you stay away from these nasty resellers/keyshops you're going to be fine.

EDIT:
After these years I too have concluded that Steam is heck of a lot better when it comes to multiplayer games, and the overall launcher experience with proper forum, MP and shop integration, but GOG is still the king for DRM-free and solo-gaming.

They're absolutely amazing. I moved over to Fanatical from Humble due to various issues and I really haven't looked back. Great deals, solid bundles, awesome customer support, free coupon codes during certain sales that make you want to support them, regular competitions, even cool features like custom birthday-themed gifting interfaces if you decide you want to give a friend a birthday gift from the site. The only thing that'd make them better is if they started offering a GOG option for games but hey, that's an issue with all of the key resellers.

Other than that minor gripe, they're great. They're the main way I buy my Steam games now. I even had the chance to grab Starfield for under $60, much to my delight since I'm passively giving Microtrash (and AAA games industry) the finger for increasing games to $70 at launch.

If it serves as another vote of trust (or otherwise), back in 2021, it got bought by Fandom, the same group that owns the Wikia sites.