There's a lot of inherent "difficulty" in finding a group that handles the learning curve of the fights at the same rate. Some people will take longer or need more practice than others and it can be frustrating. T5 jump in learning curve is quite large from T4 and those differences become more pronounced/ frustrating there.

As to why you may hear feedback that T5 is harder than entering T6 is that the jump in skill level from T4-T5 is much larger than T5-T6. Much of the focus and attentiveness learned in T5 is put to use in T6. If you were to have your group go from T4 to T6 (skipping T5), you would find T6 just as big (if not bigger) of a road block because the group didn't learn the focus and adherence to mechanics that were learned in T5.