Supra yes. Skyline no.
Nissan never intended for the Skyline to be sold in the US so they didn't spend the money required to make it meet our crash and emissions laws. Some people/companies have converted them to be legal in some states but it was expensive.
The Supra was sold in the US market so it met all our laws. However like any old car they have gradually been wrecked, heavily modified, or just worn out in the past 17 years. They weren't sold in big numbers either. Finding a good one now is difficult which has kept the value very high.