Most stealth aircraft are only stealthy against the higher frequency radars.
And only the higher frequency radars are really useful for interception or weapons targeting. The lower frequency radars are good at long-range detection but they're far less precise.
Early stealth aircraft designs like the F-117 and B-2 were optimized for signature reduction in the frequency bands most typically used by air defense and fighter radar sets. Newer designs like the B-21 and F-47 supposedly have "broadband stealth" which reduces radar cross section across a broader range of radar bands but details are classified.
The "stealth aircraft hate this one weird trick" conversation also has to contend with jamming, because many of the techniques that work great at spotting stealth aircraft in peacetime are especially vulnerable to jamming. This makes them far less useful in an actual conflict.
The Saab Gripen is said to be particularly effective in this regard.
I'm pleased at the discussion in this chain of replies, as it illustrates the cat-and-mouse game stealth (low observability?) is. Stealth never was a perfect defense, as it's detractors claim. It forces the adversary to make tradeoffs, such as increasing the costs needed to detect such craft, which can then be exploited for gain.
Jamming is a function of power and there are few radars that put out more power than the low frequency surveillance radars. Duga was colocated with the chernobyl nuclear power plant.