Here's why you hear that:
1) Unless you're spending hundreds of dollars on a Fluke, your ohm reader is going to have a margin for error. If your coil reads .8, it may actually be .5. That's a pretty normal margin for the meters we tend to use.
2) It's usually recommended to stay at or under half of the continuous amp rating of your battery, so in this case, you want to stay at or below 3.4A, which means 1.2 ohms or so on a fully charged 4.2V battery (which really means 1.5 ohms - remember that margin for error on your meter?). Maximum continuous discharge rating is a MAXIMUM, not "you can safely run it here regularly."
3) The NCR 18650B is a protected Li-ion battery, not a high drain IMR (safer chemistry) battery. If the protection fails and it goes thermal, it will explode. You don't want that to happen in your hand or near your face. These batteries are borderline ok (but not recommended) for regulated mods that have their own built-in protection for redundancy, but in a mech, you're literally playing with fire. They aren't designed for high drain use scenarios like sub-ohm vaping.
And finally, 4) Life is unpredictable and you never know what's going to happen. I recently read a post from someone who fried a battery just taking the battery cap off of his mod. A spring came loose somewhere inside the device and managed to short the battery. That's the kind of thing you can't control, and it doesn't matter what resistance your coil is or how carefully you did your math. Sometimes strange things just happen. You want to make sure you're prepared and/or protected when they do.
The biggest key to sub-ohming safely (besides only using high drain IMR batteries) is margin for error. You have to leave yourself LOTS of room to cover mistakes, inaccurate equipment, and/or anything else that may come up. Running 5.25 amps on a non-IMR battery that's only rated at 6.8 amps is not NEARLY enough headroom.
If you want to start experimenting with mechs and you're going sub-ohm right off the bat, do yourself a favor and order a few Sony VTC5s. Those have a 30 amp rating, which will be plenty of headroom for your .8ohm coil, even if it's really .5 ohms. And if you decide later that you want more, it's still enough headroom when the coil you measured at .6 ohms is actually .3 in the real world (which would be 14 amps at 58.8 watts on a fully charged 4.2v battery).
Does that make sense?
