salamanders can move and shot, they use the fire template that usually shot more models, if they make even a single wound the wounded unit needs a panic test, they have flaming attacks. Finally, they're the only "flame thrower" in the lizardmen army. The only benefit of razordon is their stand and shoot, but this increase the possibility of misfire, and we already have enough models who can shot at the enemy. They could be nice, but they cost like salamanders, and salamanders are epic. Personally, i don't like the randomness of the number of shots.
Because Razordons are not a good unit. Their damage output is unimpressive, they cannot march and shoot, and being forced to stand-and-shoot severely limits their tactical utility. Salamanders are far superior is almost every way. Anytime I would consider using Razordons, I would probably just take blowpipe skinks instead.
In my opinion Razordons have one real purpose. Protecting Salamanders. So Salamanders AND Razordons is okay. There stand and shoot combined with their combat prowess can handle most enemy skirmishers and Fast Cavalry. If it's a choice between Salamanders OR Razordons go with Salamanders every time.