I assume they were good at achieving their goal but to me it seems like japanese fencing is much less "agile". AFAIK both Europeans and Japanese used cutting swords on battlefields as a secondary or noble's weapon (rarely as a primary one) and for duels with little to no armor so it's surprising to see differences. I'm not knowledgeable enough but just looking at lots of youtube videos it looks like HEMA techniques involve more agility, being agile, parries, multiple guards etc. modern fencing is an evolution of that while kendo/kenjutsu is more static, assumes significantly different position, doesn't play around with so many guards. This seems to be the case whether it's modern sport context, more "realistic" ruleset (like HEMA), or kata/presentation of historical techniques.
There are lots of myths how katana was actually legendary magical sword or opposite ones how they broke very easily and were awful. AFAIK neither is true and common sense tells me that something used for centuries must've been at least good enough at doing it's job. Was that task different than for European swords?
If we compare the most similar weapons - swords held in two hands, i.e., longsword and katana - there is a lot of similarity in techniques when we compared HEMA (historical European martial arts) and JSA (Japanese sword arts). The basic stances are almost the same, the basic cuts are almost the same, and thrusts are almost the same. Also, the basic parries are almost the same. There are some differences due to the different lengths and weights of the weapons, straight vs curved, and double-edged vs single-edged (although these last two mostly affect advanced techniques rather than basic techniques).
The similarity in basic technique should be expected, since European humans and East Asian human are fundamentally the same biomechanically, and will move in similar ways with a sword held in both hands. The basic stances/guards (e.g., high guard, middle guard, low guard, hanging guard), basic cuts, and thrusts can be readily discovered by a pair of training partners playing with wooden swords or sticks. There are perhaps some less obvious points about parrying/blocking that can make good basic technique harder to discover, but it is still not-too-difficult. (Thus, it is unsurprising to see also largely the same techniques in Chinese sword arts using a sword in both hands.)
Why then do HEMA and JSA look different? The things that are seen are not so much the techniques used, but (a) training, (b) public demonstrations, and (c) competition. Publication demonstrations (b) will typically make use of elements of (a) and (c), and need not be considered separately.
At first glance, there is a large difference in training methods between HEMA and JSA. JSA has a traditional dependence on formal two-person kata (forms), and HEMA training will use a range of solo and partner activities, with partner activities including short pre-arranged drills and sparring. HEMA pre-arranged drills are equivalent to JSA kata, and we can compare them meaningfully.
JSA kata can be performed slowly and formally:
and this doesn't look much like the usual things seen in HEMA training. However, done as partner drills for training rather than public demonstration:
https://www.youtube.com/watch?v=n9w1QpLdXns (still a demo, but done faster and more fluidly)
the pacing and range of technique is not that different to that seen in short HEMA drills:
Koryu kenjutsu ("koryu" = "old school" = pre-Meiji schools/styles, "kenjutsu" = swordsmanship) tends to lack sparring. Some koryu schools spar, but many have no sparring at all in the formal component of their training. Today, kendo serves as a sparring exercise, and it is not usual for kenjutsu practitioners to also do kendo. Kendo is strongly influenced by the rules - this includes both the limited target areas and allowed techniques, and the requirements of scoring. Since these rules are quite different from typical HEMA rules, kendo can look very different. In addition, a shinai (the bamboo "sword" used in kendo) is much lighter than a sword. In particular, the scoring requirements can make the pacing and tactics look very different. This isn't always the case, and although kendo will show a smaller range of targets and techniques (due to the more restrictive rules compared to HEMA), the pacing and tactics can sometimes be similar. You should be able to see some similarities between
and
Some kenjutsu spars, notably Toyama-ryu (not koryu, but a modern style synthesised from a number of koryu styles). Compare
and
One of the HEMA drills/demos above (by Guy Windsor) included throws. Depending on the particular rules in use in a given HEMA tournament, throws might be allowed. Throws and takedowns are not allowed in modern kendo, but were used earlier, as see in the footage of late 19th century kendo:
which also includes weapons such as the kusarigama among the swords.
Finally, to further illustrate the impact of rules on what competitive fighting looks like, consider these two cases of escrima/arnis/kali competition. First, continuous fighting which isn't stopped when a point is scored, but instead the fighters continue to hit each other in order to obtain a higher score through more hits and dominating the fight:
Both fighters work to hit the opponent, and avoiding being hit in return is secondary (which can be argued is rather poor swordfighting). In comparison, fighting which stops and resets when a point is scored looks very different:
even though the underlying martial art is the same.
The lesson is that some caution is required when trying to understand old martial arts, whether armed or unarmed, from what we can see in modern competition and training.