How was Aku no Hana's rotoscoping received by other animators?
There's not much information out there, perhaps because it is bad practise to criticise fellow animators. But there is information from the people who were involved:

Firstly, The mangaka was satisfied with the way the anime was animated, and the studio itself was happy with the outcome, wanting realism without making it completely live action and wouldn't be opposed to doing it again with another series, mentioning that Mushishi would be a good candidate for the style.

Vertical Inc., who publish the manga in English, have also stated their support for the stylistic adaption - Vertical Inc. Tumblr

The industry as a whole isn't opposed to rotoscoping in the industry, Kuuchu Baranko also implemented it for one character's appearance and there was little outroar on the internet



Kids on the Slope also used the technique for animating the characters playing instruments, most notably the drums;



It also wasn't the target of much negative feedback either.

Personally, I think that if a large unconvential stylistic choice was made for an entire anime, there will always be negative feedback, and that makes a lot more noise than positive ones.