I use mechs and RDAs exclusively. Most of my builds are at .21 ohm, unless I am doing something unusual or building a deck for 3 or more coils. 3-coil RDAs like my stillare v3 or onslaught are built at .15. With an extra coil, though, more power is needed.
just using ohms law at maximum possible battery charge:
.21 ohm at 4.2 volt = 84 watts
.15 ohm at 4.2 volt = 117 watts
Even if factoring in for voltage drop across devices, a lot of watts are being used. But my RDAs have great airflow and I often use wide-bore top caps, so an overly hot vape or burning wick is never an issue.