I am a proponent of rad weapons. They always have been a attrition weapon and with the new rules they still are.
The changes to seriously injured rules basically made most weapons better and rad weapons benefit. The only weapons that are not affected are web weapons
Web was a overpowered trait and instead of making it less useful GW decided to make every other weapon better...
Rad phage hands out flesh wounds just more effectively than standard weapons like it is supposed to.
These weapons are always viable you just need to know what to expect. Take the rad gun and rad cannon the "common" rad weapons. They are in effect two weapons for the price of one. Firstly they are a S2, meh, AP -2, pretty good, weapon. They are either a Template or Blast with thier targeting perks. Secondly, and far more importantly, they have a 50% chance of handing out a flesh wound to anyone under the template/blast regardless of if the cannon hit or if they were wounded to start with and the Rad phaged trait itself ignores any armour save they have. There is a good chance that a opponent will loose a point of toughness without a save. Aside from goliaths and ogryns on average the typical fighter has T3. If they are now T2 as a result of a rad cannon hit then those little S3 pew pew lasguns your teks and subteks are sporting are suddenly much more threatening now wounding on 3s. A cheap hot shot pack and they now wound on 2s.
A cheap rag gun or cannon, and they are compared to the other tactically similar options the radgun analogue is the flamer not the plasma gun, and your whole gang suddenly becomes nastier.