The upside of small tanks is they don't take up as much space. The downside is if something goes wrong, it happens much faster than in a larger body of water.
Tap water varies from location to location, so while some people may do well with tap, others don't. When I used tap, my neos would grow sick and die. When I switched to RO, they took off.
Some people are able to get away with rarely doing wc as mentioned, although I'm confused why repsa would top off with tap water of 14 gh, 13 kh and never do any wc? When the water evaporates, the minerals are still left behind, so each time tap is added for evaporation, more and more gh, kh and TDS is added.
Repsa does bring up some good points about wc being done quickly can be problematic though. Also about temperature swings being dangerous.