Are you growing them indoors in pots or outdoors in the soil ?
I have grown Shikuwasa seedlings in pots.
Doesn't seem too sensitive, seems reasonably tough.
At least on the same level as other mandarin rootstocks or sweet orange rootstocks.
Doesn't seem particularly prone to root disease problems.
I had some spare seedlings that I used as Microcitrus hybrid rootstocks, seems fine.
However I am not in a cold winter area. We have frosts maybe minus 3'C to minus 5'C.
Frosts are getting rarer over recent years.