Its not that ratlings are too silly its that they dont really fit into the game in my opinion. Ratlings and ogryns are very stange in a group of xenophobic humans and while they might have human origins they will still be regarded with suspicion and distrust amongst humans. I would prefer if the players in my group played humans with similar roles (sentry or brawler for ogryn and sharpshooter or weapon specialist for ratlings). I never liked either of these abhuman races as they just dont seem to fit in with the imperial guard. THe only exceptions I will make is if they were playing a squad of abhumans or if they played ogryns with catachans as there are ogryns on catachan.
Ogryn can fit quite well into regiments from Death Worlds (as you mentioned) but also Feral and Feudal worlds. A Feudal world where the humans drug ogryn brutes and 'sacrifice' them to the Imperium as their tithe could be fun. Likewise, ogryn are typically very pious and would do well in regiments raised under the Penitent homeworld. Do be careful not to put them into Mechanised Infanty regiments or any other where vehicle use is mandatory (Armour, Artillery, Drop Troops, etc.), since they don't typically do well with enclosed vehicles.
Ratlings, OTOH, can really do well coming from almost any of the more civilised worlds. Agri-worlds and Frontier worlds would work well too.