According to the data sheet for the Scotchlok™
Insulation Displacement Electrical
Tap Connector 560:

Quote
18-16 AWG (0,75 -1 ,0 mm2) solid or stranded, 14 AWG (1,5 mm2) solid or
stranded, tinned or untinned copper conductors. For use with thermoplastic
insulated wires such as; T, TW, TFF, TFN TFFN, AWN, TEW, SAE-GPT, with a
maximum insulation diameter of .145 inches (3,5 mm).
Maximum recommended current for general purpose applications: 18 AWG –
7 Amps, 16 AWG – 10 Amps, 14 AWG – 15 Amps.
Engineering / 3M™ Scotchlok™ Insulation Displacement Electrical Tap Connector 560B is
Architectural capable of connecting a tap wire to a run wire in the range of 17-16 AWG
Specifications (0,75-1,00 mm2) solid or stranded, and 14 AWG (1,5 mm2) stranded, tinned or
untinned copper conductors.


It says they do work with solid wire. This is for the blue ones; the red ones, like the one I saw, are designed for smaller-gauge wire (18-22 gauge wire).

Perhaps the biggest problem with these Scotchlok connectors is that you have to use the correct size for the wire you're tapping into to make a reliable connection. If you don't, expect problems.