I think you've been misinformed, or are taking a "general" stance on the subject. The Efest purple series are built for high amps. Their 18350 is good to 10.5A! I just rebuilt my Origen last night, and it came out to 0.41 ohms, which equates to 10.24A, which *could* be run on an 18350. However, it's a best safe practice to divide the amperage rating in half and not exceed that, which in this case would be 0.8 ohms - still sub-ohm, and very safe (with the Efest purple battery - others, not so much).
As to your 18500 comment, the Efest purple 18500 is good to 15A, which would be a 0.28 ohm build, so sticking with the 1/2 amperage safety rule, you could safely build a 0.56 ohm coil on the Efest purple 18500.
Now, just for the record, I always use 18650's on my sub-ohm builds. It's not just safety, it's a guarantee. My 0.41 ohm build is pulling just over 10A on a 35A battery - less than 1/3 of what it can do. Besides, with sub-ohm sucking so much power, it's nice to have 18650's.
http://.........blogspot.com/2014/08/efest-battery-blowout-all-shapes-and.html
EDIT: I forgot that ECF is censoring links to V apor J oes, so here's his link to ebay, where you can see photos of the amperage ratings of each battery. Why ECF have to act like such children is beyond me.
Efest Purple Battery 18650 35A 18500 15A or 18350 10 5A All 3 7V Flat Button | eBay
Agreed. It would fall within the realm of safety (given a good battery such as the Efest purples), but you'd better carry a 4-pack of spares and like changing batteries!