I made my own butted mail shirt (buying the rings was cheaper than buying the finished article), and can make a few observations;
* A tightly-cinched belt around the waist is a must (pull it until he says, "Jings!" then use the next hole) with the shirt bloused above it - otherwise you're lifting the entire thing when you breathe - this gets you very tired, very quickly.
* You need a REALLY heavy sword- or axe cut to actually penetrate chain-mail - it doesn't do much for impact protection, though, so an arming jerkin beneath it is a must. Fatal injuries can still be caused by a hard enough blow if there's no padding.
*Piercing between the rings is possible, but you need a very narrow point - even a thrust from the wrong type of sword can skid off.
* An arrow or spear-head which is too broad is more likely to break than penetrate the armour - although the likelihood of knocking your target down with such a weapon is fairly good. The roman pilum was designed for this, as were bodkin-point arrowheads. Balista bolts, of course, generate so much force that they're pretty unstoppable.
Chain-mail never actually went away from the Roman army, and was always the commonest type. After the Varus massacre of AD 9, three legions had to be equipped very quickly to replace the lost ones. Segmented armour can be made much more quickly than chain, and with the soldier having to purchase his own kit, second- or third-hand equipment became common.
There's also artistic license. Artists made sculptures, friezes, reliefs, and murals of Imperial campaigns (propaganda) and soldiers in chain-mail could be anyone. The distinctive segmentata was unmistakeably Roman, allowing the plebs to easily see that the Romans were winning.