Being a cynic I suspect the reason dealers are against prewound is that Janome loses business. Your dealers may be hearing that prewouds are no good from Janome headquarters.
I've never had trouble with those from Superior Threads (I use Bottom Line mainly, Masterpiece when I want cotton on the bobbin). I've never tried any other prewounds.
Bob tells you on his site which machines do and do not take his bobbins.
The one shortcoming is that neither of my Jems take Bottom Line top and bobbin. My Bernina does. but won't take the prewounds, so have to rewind one onto a Bernina bobbin.