I don't think the breakdown is so much "metal vs. plastic" as it is "light duty vs. heavy duty."
To the OP, those plastic "blitz" cans that you buy at the hardware store are designed to hold fuel for your lawnmower, snow blower, etc. They're perfectly adequate for that use and there's a reason you can get them for $20 or less.
But throwing one of those in or on a vehicle for an extended trip where it's going to be exposed to extremes of temperature, sunlight (which can break down cheap plastic) and so on is asking for trouble, because a can like that is more likely to fail under those circumstances. Now, by "fail" I don't neccessarily mean it will break open, but it could leak, vent vapors, etc, all of which could be bad depending on the situation you're in. I had a cheap blitz can that I took off roading when I had my Tacoma. With the pickup, it worked fine because I kept it in the bed and when I stopped for the night, I took it out and set it on the ground. However, when I switched to an SUV and had only one "cabin", I realized very quickly that unless I had some way of mounting it to the outside of my vehicle, the fumes would be unbearable in very short order and potentially dangerous.
You can't lay cans like that on their side because they leak and if you mount them outside, heat will cause them to expand and then the cool night air will cause them to contract, again this increases the leakage and vapor problems.
After a couple of trips where I tried various things like wrapping the cans in multiple black plastic trash bags, I finally broke down and got some decent plastic cans made by RotoPax. RotoPax cans are red plastic like the blitz cans but at that point the resemblance ends: The RP cans are very heavy, thick plastic and are extremely tough. They are designed to me mounted on the outside of vehicles and expansion/contraction/vibration affect them very little if at all. I had a set of RP cans on the roof of my 4runner on the last 2 DE trips and they didn't leak a drop, despite being mounted on their sides and despite the fact that temperatures ranged from the high 30's to the low 90's.
The only negative to the RP cans besides price (3 gallon cans were $80/each) is the stupid CARB spouts, but that was easily cured by the aftermarket (I guess legally they have to sell the can with a CARB compliant spout but they can then sell you, separately, a non-carb spout that works much better.)
The NATO Scepter cans are also much, much, MUCH thicker plastic than the Blitz can. I've used those a lot in the Army and never had any problems with them as long as the lid was screwed on tight.
As for metal cans, I don't really have much experience outside the military with them. I know lots of people run the slick looking green NATO cans and like them just fine.