While I suspected I didn't have the resources to obtain UL certification for the XTB or XTB-II, I did pursue the possibility.  Unfortunately, the expense is even higher than I anticipated, and well beyond the scope of this project.  The volume is just too low.  Even if sales of the XTB and XTB-II increase by 100%, I would still have to raise the prices of each unit by $100 to recover the cost.  Unless production of the XTB or XTB-II is transferred to a large equipment manufacturer, UL certification is not feasible at this time.

I am not aware of anything that would prevent UL certification of either unit.  The safety components, such as the transformer and flame-retardant case, have UL ratings.  All components that see full line voltage are conservatively rated.  Line voltage capacitors are rated for 160VAC in the XTB, and 200VAC in the XTB-II, which has more room for the higher voltage components.  The international version of the XTB-II is supplied with UL rated 250VAC capacitors.  Both units are fused to provide protection should either the isolation capacitors or MOVs fail.  The transformer is impedance protected, and will open-circuit if there is a major internal failure.  Special care was given to the printed circuit traces carrying full line voltage to provide maximum possible trace separation.  If there is a major electrical failure inside the XTB, the flame-retardant case is designed to prevent that from propagating beyond the unit itself.

So, while the XTB and XTB-II are not likely to have a UL rating in the near future, both have been designed with operational safety in mind.  Like all electrical devices that can dissipate heat, it is good practice to keep particularly combustible materials away from either unit.  Signing the liability waiver is your acknowledgement that you accept responsibility for the proper operation of the unit.