That is a misconception of you use less nic with them, in fact given the math you can use equal up to 3x or even higher the nic in a single hit compared to a tank like a nautilus.
1:1 Nic usage we will use the Nautilus at 1.6ohm at 13watts using 12mg as the baseline (1ml per 30minutes)
1:2 Nic usage, an RDA at 0.6ohms at 25watts, you would need 6mg (2x6=12) (2ml per 30minutes)
1:3 Nic usage, an RDA at 0.3ohms at 50watts, you would need 4mg max (3x4=12) (3ml per 30minutes)
1:4 Nic usage, an RDA at 0.15ohms at 100watts, you would need 3mg max (4x3=12) (4ml per 30minutes)
1:5 Nic usage, an RDA at 0.15ohms at 150watts, you would need 3mg max (5x3=15) (5ml per 30minutes)
Higher in wattage you go up, more juice you burn through vastly more juice, figure out how much juice you are consuming in a given time period, 1/2hour to hour and you can kinda figure out how much nicotine you are using in that time period, Nic %, ie 1.2% per Milliliter is 12mg per Milliliter.
Now, with RDAs, anytime you are juice direct to coil and a single, well flowing wick you get direct flavor, the higher the wattage more you vaporize, more you vaporize flavor, vapor production, and nicotine throat hit increase exponentially like example above. Yes they are less convenient compared to a tank when out and about, been when at home just relaxing and swapping flavors, you can't beat them, that and if you are chasing clouds, not many tanks can keep up with an RDA when it comes to fogging a room, nor the intense flavor and RDA can bring out, letting you taste every nuance in a flavor at inhale and exhale (front tones/back tones) and subtle flavors muted out by a tank system.