It looks like they were well executed by somebody who did some research before their build.
In my mind what kills the cabs is not the homemade DIY aspect, it's the personalized interpretation of adding the horn enclosure to make one big cabinet. That kills the versatility of an 825/828 cabinet, and prevents you from physically adjusting the phase alignment of the LF and HF sections when modifying, or otherwise experimenting with crossovers.
Additionally, an 825/828 box is already rather over-sized in terms of handling/moving. Why anyone would want to augment their hulk with the addition of more cabinet is beyond me.