He wasn't the right manager that Arsenal needed. Arsenal needed to depart completely from the Arsene Wenger era were the defence had been reduced completely to rubble - Emery however was probably acquired to continue that stlye despite the fact it had already been shown up.
His reputation has certainly suffered at Arsenal, a lot of it deserved. Asking someone like Sokratis to stand on the corner of the 6 yard box to receive goalkicks for example is seriously negligible management.
However, there's an awful lot to be said about the Arsenal playing staff and boardroom that this guy was tasked with taking on, the players didn't perform for Wenger who had virtually built that club so they were never likely to listen to Emery. Add to that, Arsenal have a boardroom whose entire reason for being is to fleece their own supporters out of every single penny they can get away with for their own gain.
I still rate Emery, winning 3 Europa leagues is a serious achievement that cannot be understated. I just don't see him as the right man to lead us though, I think a lot of the problems he encountered at Arsenal, and was unable to deal with, he'd encounter here as well and he wouldn't have the attacking qualities of Lacazette and Aubameyang to bail him out.