This answer is one of those, Well, it all depends ... answers. All-in-all, I have heard good things about the newer electric impact devices. With improved battery life, they last a long time between charges and can produce reasonable amounts of torque. There are some considerations.
The main thing you need to look at in these tools is what is the amount of torque it can turn. If the maximum amount of torque produced is less than the job at hand, it's not going to work (obviously). All impact tools are going to have a torque rating on them. As that torque value goes up, so does the cost. Air tools have a maximum they can produce, but are also affected by the amount of air pressure and the volume by which the air can flow (what can the compressor sustain).
With the electric impact, you'll need to look at how much voltage the battery pack has, and how long will it last (milli-amp-hour or MAH ... could also be just Ah or amp hour rating). Again, as amp hour ratings go up, so will the price as well as the weight of the tool.
Also ensure you look at the ratings on the piece. Amazon.com customers are pretty good about leaving ratings, but beware of the outlier. Some people, because they have bad experiences with a delivery of a product, will write a scathing review which really has nothing to do with how a product performs (don't just rely on the start rating, but read what is actually said).
Lastly, as you are suggesting, never utilize an impact to tighten a fastener. It is soooo easy to over torque the fastener, it isn't even funny. I will watch tire place mechanics utilize their air impacts to put lug nuts back on, then come back with a torque stick to verify the torque on the nut. When I get home, I loosen them all and retorque correctly. I've utilized a torque wrench to verify the torque is all over the place when they utilize this method.