LMR-400 is a trademark of TImes Microwave, so unless you are buying a knock-off it should meet the stated spec. There are various clones, but they usually have a similar, but different name.
I’m not sure it’s really a prejudice, rather than it being a matter of using the right tool for the job. If you have a system that is 50 Ohm, then using 50 Ohm co-ax is the optimal way to do it. A 75 Ohm coax will work, with some varying penalty to performance depending on what quality coax you use. As with most things, there is a tradeoff between performance and cost, and it is up to the individual to decide where the balance lies.
Also, the input impedance of various rtl dongles depends on the manufacturer. The originals intended for use as TV receivers are likely 75 Ohm, but certainly the rtl-sdr.com variants are nominally 50 Ohm inputs, as are the airspys.