Check out the HBRpowerwall guy on YouTube. Also jehu garcia on YouTube both have extensive information in their videos, and frequently answer questions about powerwall builds. I personally do not have any experience with powerwall builds, but though your question deserves an answer.
My "take" on 18650 batteries is that they have the potential to hold a relatively "large" amount of energy, for their size. People refer to this as, "energy density." Anything with a high energy density can be dangerous, like gasoline or acetone for example. Please don't take my word for this do some research. The main "danger" is from faulty cells, overcharging, blocking the vents in 18650's, and not having enough cells in parallel to share the load you are intending to discharge. So not overcharging 18650's is crucial. Having enough batteries is parallel is crucial. Other than that, explore the videos on youtube I suggested.
These kits are really well thought out, and well designed to fit many different applications.
If I was building a powerwall, which I am NOT I would consider the following:
Mounting it "outside" in a weather proof location with proper venting and not being too hot or too cold. Also I would mount a smoke detector, have proper fusing, breakers, and disconnects. A lot of these powerwall guys are mounting their batteries onto plywood, which is flammable is, in my humble opinion a poor option. Just look at any UL listed enclosure for electronics, and you will find it made of metal, with appropriately formulated high temperature plastics.
Lastly, having a fire suppression system on hand SPECIFICALLY for electrical fires would be a smart precaution.