Personally I'm a fan of Conf 3 in the A321, assuming you've got enough runway and a headwind. It definitely lands pretty flat, with less of a pronounced flare. It does look and feel different, though, and perhaps the risk is related to the way we're used to flying the aircraft rather than simply the geometry. The A320, on the other hand, always feels a bit wrong to me when on approach in Conf 3. I'll still use it when appropriate, but I'm not a huge fan.
I wonder if part of it is down to our reaction to the visual picture. The more 'wrong' it looks out the window, the more likely you are to go around, thereby reducing the likelihood of a tail strike. Getting towards the sort of pitch that's likely to cause a tail strike in an A320 will look far more wrong (compared to what you're used to seeing in most approaches) than the relevant pitch in an A321, certainly for those who are used to flying the A320 most of the time.
If you're used to landing the A320 in Conf Full, which I'd imagine that most folk are, then the Conf 3 visual picture is markedly different. In the A321 there's far less of a difference between Conf 3 and Conf Full. (Partially that's down to the physical flap postion; on A319 and A320 CEO and NEO the difference in flap angle between 3 and Full is 20 degrees, on an A321 CEO it's only 4 degrees, and 9 degrees on an A321 NEO). As a result, there could maybe be a tendency to allow the A321 to get closer to a tail strike scenario as your experience isn't setting off quite so many alarm bells approaching 7 or 8 degrees nose up as it would approaching 9 or 10, by which time you'd almost certainly have thrown it away and safely gone around. In the heat of the moment, you're probably reacting more to the picture out the window. It's a visual manoeuvre, after all.
You're also going to be landing much faster in a typical A321 with Conf 3 than any other configuration across the whole A320 series. Maybe this leads to a higher tendency to experience ground rush, shying away from the runway and overcompensating during the flare, thereby increasing the risk? Then again, flaring late and bouncing after an initial firm touchdown is also going to be harder to recover neatly in an A321.
Anyway, just some ideas. It's obviously going to vary for different operators depending how frequently you're flying each of the different variants. They do all react differently; CEO vs NEO, WTF vs Sharklet, 319 vs 320 vs 321, etc...