Its ordinary nature doesn´t rely exclusively on that. It could have managed to cause a greater impact like a seemingly boring place like the Crystal Empire and its crystal ponies.

Just that the Crystal Empire is memorable because of how many episodes it has appeared in the show, how different the setting looked in comparison to the rest of Equestria and how it has gained slowly more spotlight with different plots (Equestria Games, Sunburst, Thorax appearance, Flurry Heart...). The location might not be the most interesting one but it slowly displayed more layers to its track record.

But the problem doesn´t end there because the concept of the rainbow isn´t exclusive to that place. Before getting there, we had seen Cloudsdale and Rainbow Falls before in the show. So this means that its entire lore depended on the concept of bringing happiness and harmony to the town by recoloring the entire place. Its lore was also the entire point but it happens that this plot device was seen before in many episodes so it overlaps the whole thing to something not only familiar, but also overdone in the past. If there were more layers of lore behind it, it would end up with much more interesting results.

So, what additional issue does Hope Hollow have? Basically the single fact that the special was contained there: only in the special and nothing else despite expanding the map. None of the characters which were seen there appear in the finale nor in any other way in the show (or even the comics so far) so their impact gets greatly reduced. The location is summed up in that single special. If it appeared (or any of the new characters) in another episode in the show (like Mount Aris/Seaquestria did after their debut in the movie), then they would have received more impact or a bigger legacy than this.

This last paragraph makes the huge difference in the end. People might have considered The Crystal Empire boring in its debut but over time, it escalated positions silently. Hope Hollow didn´t receive that treatment beyond its debut (and it came really late), there weren´t any more secrets or reasons to discover it beyond its main conflict.

And so, the lack of lore or uniqueness entailed to review the entire thing directly from the interactions or characters themselves or the Slice of Life story exposed for one hour. In that regard, the reviews were written in the S9 thread, I am not going to repeat them here.