Let's say that both over the years have earned a reputation as excellent studio pianos; they are "workhorses," simple but efficient, with good actions, but they are still instruments that fall into the economic range, so defects are not lacking. For example, their sound, no matter how much tuning interventions are made on the hammer rail, will nonetheless remain a "standard" sound, not very personal, not very European (you can change hammers but it costs money); in fact, those produced for our market are born with a different timbre compared to those for the Eastern market, which is tendentially brighter
Years ago I bought a U3x to put in many hours of practice, and I must say that as a piano in terms of action it wasn't bad, even if a bit heavy, but over the long run the sound turned out to be monotonous, impersonal; in short, in my opinion, the Japanese are good, but they come from a different culture, and it's a bit like when Westerners try to emulate and adopt Eastern culture (like Italian Buddhists), basically they try but they lack the ingredients.
Anyway, Kawais are said to be tendentially "warmer" with lighter action.........
All this is in the economic range; instead, from the C5 onwards, the cost changes and so does the music.
I prefer TOYO (Apollo) over Yamaha in the budget range.
Then I'll say one last thing, in my opinion an old Schulze is much better Pollmann, Anelli (Cremona), Zimmermann, Schiedmayer, Feurich, Bluthner..... without bothering with the big names........ I replaced the U3 with an old Grotrian Steinweg and there is simply no comparison. (then there's my beloved Pleyel, but that's another story)
P.S. I have nothing against the Japanese, I just want to say that alternatives are abundant!
ciao