Another ‘I’ve got this friend’ annecdote? The number of these you have produced over the years is truly staggering. I am calling BS on this Taemien. For one as @LostBrythunian already correct pointed out, there is no way to reliably and consistently obtain greater pets on the Exiled Lands. It is purely governed by RNG mechanics. While shadebloom will increase the percentages, there is no guarantee, there is no reliability. Period.
Moreover, while it may be easier to obtain say a baby shaleback or crocodile, I would make the following counterpoint. It takes more skill for a new player walk to the Highlands, Frozen North or Savannah, procure the 50kg animal then wander home than it does a T4 thrall. The simple fact of the matter is there are guaranteed T4 thrall spawns at coordinates E3 and I4 in the River biome. They can be knocked out but naked using only a wooden truncheon and 0 points in Authority in <3mins. And that is to say nothing of the ease obtaining mid to high tier thralls such as brutes, relic hunters, berserkers, Thugras, N’abatus, Sullys, Teimos’, Lians, the list goes on (and I did not even cover guaranteed archers in that list). Greater Pets on the other hand require collecting the pet, running a dungeon, killing 4 three-skull bosses within, and then fighting the RNG mechanic to obtain them. The risk for obtaining a T4 thrall is as low as a snakes belly, and while far from substantial, the risk of running the aforementioned loops to obtain a Greater Pet surpasses it.
You want to discuss the risk-reward coefficient?
At this point I am going to say the generalisation that obtaining T4 thralls being more difficult than Greater Pets is debunked, and claiming otherwise is a false dichotomy.