Generally, that's true, but compression technology is evolving, making it possible to fit more and more into the existing bandwidth. The Univision/Unimas stations are running two 1080i streams, plus two 480i streams, and their programming often includes soccer matches. A professional can probably spot the problems with that, but I doubt most untrained viewers can tell the difference, especially if they've been used to watching heavily compressed video on cable or satellite systems. A lot of it depends on the quality of the multiplexer as well. K38IZ-D has serious problems with many of their seven feeds, most likely due to older, cheaper, less capable systems.
ATSC 3.0 is promising even better compression technology, making it possible to air UHD content, which is not feasible under current ATSC 1.0 technology.