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Parametric instabilities have been long known as a major space-charge halo mechanism and studied intensively. With realistic distributions, for the first time we confirm that high-order parametric instabilities other than the envelope instability are not manifested in actual linear accelerators. Moreover, the effect of the envelope instability is weaker for the realistic distributions than for Gaussian distribution: onset of the envelope instability is delayed significantly and the emittance growth is reduced. On the other hand, the effect of the fourth-order particle resonance is stronger for the realistic distributions. In actual high-intensity linear accelerators, parametric instabilities can be disregarded except the envelope instability. One needs to heed only the envelope instability and particle resonances. [Jeon et al, Nucl. Eng. Technol. 58, 104379 (2026)]