After having a couple of lockups already in the first few days of using an X-T3, I have to wonder: is this a firmware issue affecting all units, or a sample defect that could be cured by exchanging for another copy? Has anyone done that successfully, and used the replacement body the same way without further trouble? (With any camera that's had this problem, including H1 and T2?)
For me, lock up problems have occurred randomly --- where I had a problem is when the lockup occurred frequently, which for me would be more than once per week or month.
Lock-ups seem to happen more frequently when I'm shooting multiple bursts or lots of single shots closely spaced.
Using newer SD cards from reliable brands and reliable sources (i.e., no fakes) helped significantly.
In high-performance burst mode with the battery grip, using Fujifilm branded NP-W126S batteries helped.
Lastly, firmware updates also reduced the number of lockups.
Assuming you are using good memory cards --- swap them out even if you are convinced they work fine in other cameras (i.e., aside from Fujifilm, there aren't that many UHS-II compatible cameras out there) --- and genuine Fujifilm batteries and have the latest firmware, if you get a lot of lock-ups, it wouldn't hurt to swap camera bodies.
I am not sure if Fujifilm's repair centers have the test equipment to thoroughly test out the memory card interfaces at the highest performance speeds. So just turning in your camera for repairs may not be good enough.