I think cancellation unequivocally answered that one.
It is up to the fans to decide how successful they were or weren't.
Well I think it is a more complex answer than simply arguing the show was cancelled. Afterall there are good or promising series that are canned while subpar stuff goes on for years and years.
I'd also point out that while ENT had a rough first two seasons the show had a marked improvement in seasons three and four. Sure Coto pretty much guided season four but season three was a lot of their vision and played strongly into Brannon's strengths-high concept sci-fi elements, big spectacle imagery, action and plot-driven storytelling.
I think a large part of ENT's problems were the writing and the characters not resonating with the audience. And even the oft-cited savior Coto couldn't really make the characters come alive so he instead focused on the stories and continued the plot-driven approach.