As player characters level up, game management becomes more complex. At first this can be fun because the DM can present more varied encounters. But eventually it becomes onerous -- balancing encounters becomes very hard and a real crapshoot at times. PC's have multiple attacks and so many abilities combat can slow down to a crawl. The DM can no longer anticipate when or how a PC may be able to break the plot. Opinions vary as to when the tipping point is reached, but general consensus is that the problems become acute sometime after 10th level. It does not surprise me that Tier 4 (Levels 16-20) is among the least popular answers in your poll.