He has a point, but there is a thing you can do to offset some of this: read all of the comments. Plenty of times there will be someone in the comments pointing out what's wrong in the videos.

The problem with Lithium Ion installations of any size is that the batteries are inherently dangerous and mistakes can easily set you back more than you saved on the whole thing. Unlike say your car where as long as you stay away from suspension, brakes and steering the worst that could happen is that you end up stranded by the road side.

Another issue is that when it goes wrong with Lithium Ion it can go wrong fast and it isn't easy, or sometimes even possible, to stop a thermal runaway once it gets going, especially with densely packed cells, besides the fact that you likely won't be watching your batteries 24x7.

There are some people on youtube that advocate for extremely unsafe practices (Jehu Garcia, for instance) and that have a huge following, so Will Prowse is certainly not the worst that you could run into there. It would also be good if he indicated more clearly what his sponsorship relations are and how that relates to the advice that he is giving, quite a few of his videos feel like thinly disguised ads.