People say sub-ohming is for more experienced people because there are certain battery "implications" that come with vaping at below 1 ohm. The amperage capabilities of the battery primarily come into question, and the problem isn't really with .5 ohm builds (although if you have a battery with a 10A limit, that might be a problem). The problems really come with people rebuilding their own coils at insanely low ohms. Below .22 ohms exceeds 20A, which is the CDR of most 18650 batteries on the market. Beyond that, .1 ohms exceeds ANY CDR for any battery, 18650 or 26650, unless you're running your batteries in parallel (which is something I only recommend for the most experienced vaper). A lot of people will rebuild their coils to be well beyond the capabilities of their battery, and really they're not only ruining their battery's lifespan (the battery might only last a few months as opposed to a whole year), they're potentially "playing with fire". When batteries vent, they violently dump the entirety of their remaining nominal voltage in a fraction of a second. Hot, HOT gas gets expelled from the battery so rapidly that your mod may not be able to adequately vent it in time. The pressure difference could essentially turn the mod into a pipebomb.
But that's just a shortened version. There's a lot more details and intricacies involved in battery safety. As an owner of a regulated mod, however, you don't really have much to worry about. Your device shouldn't fire anything that exceeds the amp limit of the internal battery. Additionally, the chip should have safety features that prevent the battery from violently discharging in case you happen to short a coil or something.
Lastly, the wattage is somewhat of a personal preference. With the Subtank's .5 ohm coils you should start at around 15W and work your way up to whatever feels right for you. The airflow options kind of give you the option to go "quite airy" down to "cigarette-tight" so you can kind of experiment with the wattages at different airflows to decide what you like the most. For me, personally, I like hitting my Subtank at "fully open" airflow around 35W, using the newest vertical Subtank coils. With the horizontal ones I usually hit it around 25-30W. The vertical ones, I've noticed, can take a bit more wattage -- I think they made the juice holes bigger.
Hope that helps, happy vaping.