OK, so lower ohms = more heat.
And all other things being equal (or at least within a certain tolerance) it's that simple.
However, I'll add some minor clarifications that may help decipher this mech-hard-hit and RBA thing a bit.
1) The "roll your own coil" people use differing thicknesses of wire (the gauge). So lower gauge = thicker.
2) Thicker wire takes more power to heat to glowing. Watts = power. Like light bulbs.
So you see all sorts of ohm numbers depending on what wire they use as well as how many watts they want. There's charts all over the place but they are mostly for "standard" atomizers and cartos. Standard gauge is about 34 ga, as I understand it. It's not uncommon for RBA users to use 28 ga wire. That's not a requirement, just a common custom for various reasons.
3) The "Hard hit" thing is not only a function of voltage but of amperage. Here's where we put the above together along with a mech to get a "hard hit"
Basically 28 ga wire needs watts to heat it. Also thicker wire has less resistance per inch...so lower ohms. Now, what people do is wind, say, a .8 ohm coil with 28 ga wire or a 1.5 ohm coil with 30 ga wire. Or whatever.
But the battery and mod must be able to put out the necessary current at whatever it's voltage is to achieve the watts. So the "hard hit" from a mech comes from not having amp limits in the mech (batteries have them ...check specs) combined with lower ohms and heavier ga wire.
So...that's where you bump into limit with some non-mechs...they can't push the amps in all possible configurations that you'd screw onto them. However, with a little calculation and planning you can get a fine hit off anything. Note I didn't say identical. I said fine/adequate.
So when you check out RBA's...read up on ohms law. Also on how they are used. And in particular on battery safety and limits.