A while ago, I posted on how the inbuilt McAfee based security scanner was quite intrusive in regards to how they process their program. Their terms and condition at the time was very convoluted and suggested they syphon a lot of private data for their own use (your IP address what apps you have and how long you use them for,etcetc)
However it seems they've recently updated their terms and condition as per below:
So it seems they only use your data if you use their actual security app. However the inbuilt program seems safe to be used.
I've tested this for the last few weeks and have not noticed any issue with battery use as well.
To be honest, as long as you practice safe mobile use, you don't really need this (ie, not downloading random apps or clicking random links). However if you do sideload apps, having this working in the background is not a bad idea and gives you that extra piece of mind.
