So, I won't pretend to know anything significant about katanas.
What I know is that they were expensive and time-consuming to make, usually failed against most weapons hailing from countries that had easy access to good steel instead of having to spend a ton of time and money tempering it, that they were absolutely useless against armor, and that their main advantage was cutting power against soft tissue.
They weren't even used as a main weapon in the battlefield as far as I know (although this also applies to arming swords and the like in most cases), as japanese had the common sense to use spears and bows. So why make a weapon only efficient against unarmored people? From what I've gathered, although this might be where I'm wrong, they were able to afford armor for their main fighting force, so it wouldn't even be that effective in infighting.
Bear in mind that I really want to be corrected here, because common sense warrants that I'm wrong/missing something and Japanese people didn't invent and the manufacture a sword that was one of the most difficult swords to make in the world, simply because making quality steel from what they had required a ton of work.
So yeah, most of the seemingly unbiased sources say katanas are good swords if used the way they were designed to be used. What situation were katanas designed for?
/u/bigbluepanda in this answer tells how katanas became popular due to it being a symbol of the samurai class, relative peace of the Tokguwa shoganate and also with smiths being able to decorate and produce swords that were more like works of art.
What I know is that they were expensive and time-consuming to make, usually failed against most weapons hailing from countries that had easy access to good steel instead of having to spend a ton of time and money tempering it, ... Bear in mind that I really want to be corrected here, because common sense warrants that I'm wrong/missing something and Japanese people didn't invent and the manufacture a sword that was one of the most difficult swords to make in the world, simply because making quality steel from what they had required a ton of work.
There are a lot of incorrect things often said about Japanese steel, Japanese steel-making, Japanese sword-making, and famous weapons like the katana more generally. If one takes out the influences from these, then the comparison with other swords becomes rather straightforward.
First, Japanese iron ore was good, and they had no shortage of it, before their modern industrialisation.
Second, Japanese steel-making in the tatara furnace, with subsequent folding of the steel afterwards to get rid of excess slag and homogenise it, was just another version of a very common steel-making technology - the bloomery furnace. Swords were made from bloomery steel in various parts of Asia, and in Europe. In European, the Industrial Revolution provide new steel-making methods, but bloomery steel continued to be used in parts of Europe and America into the 18th century. For much of the world, bloomery steel was the usual steel for sword-making until guns (mostly) pushed swords off the battlefield.
Third, Japanese techniques of sword-making using laminated construction and differential hardening were also common around much of the world. Often, techniques other than the Japanese method of differential hardening using a clay coating were used (multiple methods of differential quenching, differential tempering after quenching, differential hardening by slack-quenching, etc.), but the results were similar.
The result was a sword that was a normal sword. As already noted by u/Claudius_Terentianus quality varied (and the same can be said for European swords, Chinese swords, etc.). Fundamentally, it was functionally similar to many other swords around the world. When you compare it modern industrialised sword production (e.g., British sword-making in the 19th century, with steel of consistent composition, well-tested heat treatment using controlled temperatures, modern machinery for roller-forging, etc.), then it can look a bit primitive in manufacture, and was more expensive. But for a late Medieval/Early Modern sword, it's a normal sword.
When they came up against other swords, they fared well enough (e.g., in Korea 1592-1598, and in piracy through much of East Asia). The Japanese exported weapons to SE Asia, and the buyers did not think them useless rubbish. The Chinese were less fond of Japanese export practices in the Ming Dynasty tribute trade, but there are surviving Chinese swords with those exported blades. See https://markussesko.com/2013/11/01/japanese-sword-trade-with-ming-china/ for info on these tribute trade missions, and https://www.metmuseum.org/art/collection/search/23387 for a nice Chinese sword with a Japanese blade (for more info on this sword, see Tom, Philip M. W. "Some Notable Sabers of the Qing Dynasty at the Metropolitan Museum of Art." Metropolitan Museum Journal (2001), pp. 11, 214–17, figs. 15–17, pl. 3.)