Beyond just generally enjoying their journalism, one of the journalists covering Ukraine for the Guardian is the husband of a friend of mine. So I get to see his dedication to his work from relatively close up, and makes it at least very clear to me that they're on a journalistic mission worth supporting.
I think any media outlet of that size (or any size, probably) can be criticized, and some of it will be valid. But that's just in the nature of the craft - it's a challenging field, and it's impossible not to make some mistakes. In my experience the Guardian remains committed to quality journalism, and they have some of the best people out there working for them.