It mostly comes down to your build and wicking, as you're the one in control of the vape with a mech mod. Some juices vapes better at say 13w whilst others 20w+ So one could say that's one of the downsides with a mech, you can't change the wattage on the go for diffrent juices.
Edit: Also some juice brands tastes better than others. And if the juice is newly made it would require a steeping time. So try leave it for 1-2 weeks and then try it again.
As far as safety of the juices, noone doesn't realy know for sure. The juice makers claim x is safe but in the end it has showed high amounts of diacetyl for example. (still less than a cigarette so i don't care personally.)
Acrolein is only made when the coil reaches to high temperatures afaik, i could be wrong on that. Was quite some time ago i read about the acrolein.
To be realy safe with the juices you'd be best of making your own juice. Then you know exactly which flavoring you've in your juice. Or vape flavorless (pure pg/vg blends)
Here you can read about all the studies DR farsalinos have done for the vaping community. Such as safety and acrolein you wonder about.
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But in the end, vaping isn't entirely safe. It's just a way safer and better alternative for smoking.