I think the biggest problem with GT6 is the lack of something that truly divides the stages, which results in too much complexity in the early, making it easy for beginners to give up, and the lack of content in the lategame also makes it difficult for people to continue playing.
Indeed, stacking too many levels like the GT5U is foolish, and stacking with the same functionality machines for more than 8 tiers will only make people feel annoyed. But this also means that through a tier system, it is easy to set many key nodes to clarify the development direction, so that the game process is relatively fixed and not too chaotic. Taking the circuit of GT5U as an example, although players are complaining that this system is too complicated, through some key materials and components such as polyethylene, epoxy resin, and various wafers, it is possible to lock in the predetermined development direction and determine the main game flow by “using the things from the previous stage to make the next stage”. This also makes the GT5U series still the mainstream technology modpack to this day, while the GT6 only has a small number of packs.