I don't think anything qualifies as either success or failure given the circumstances. The season was ruined by the time he took over and anything we manage to salvage from it will be a bonus. We have improved under him but it's impossible to say how much since we're lacking so much quality in key areas. We will just have to see where we're at next season.
The kind view on Arteta is that he has managed to turn around the worst funk Arsenal has been in for 30 years, has been brave and given chances to young players, has brought better structure, and has made some big moves to weed out the unprofessionalism that was seeping into the team from every direction.
The critical view is that the turnaround still hasn't been enough to make a tangible difference in the table, that we still play fairly negative football, lack consistency not just between games but also between the halves within those games, and are now getting exactly the results we're due, having overperformed heavily before the covid break.