I can tell y'all why they were frowned on in the beginning. Because it's easy for a chapter 3 or 4 city to make far, far more tier3 than they could possibly use. Simultaneously when chapter 5 was top of the tech tree it was feasible for end stage players to take cross tier trades but this did not carry forward into Chapter 6, Dwarf, or even the next chapter, Fairy. Various factors come into play that puts a lot of pressure on tier1, then tier2, which made taking tier3 cross trades very difficult for end stage players.
Back at the beginning of 2016, the end stage players made a rule: cross tier trades are bad, because at that point they were literally ruining the market. This gets corrected in chapter 9, Woodelf, if you play a balanced game and upgrade factories when the possibility opens up, because the pressure to do so keeps pace with potential upgrades.
Many players learned the 'rule' early on. And in fact, it's still a good rule of thumb early on. I hope this helps