Because general high fidelity enzymes have exonuclease activity, although the fidelity is improved, the amplification efficiency will be reduced, so when using Pfu, the amplification time is often longer than that of Taq enzyme, and Pfu Ultra is better. But it is inevitable that Taq enzyme P has bands. If you switch to high fidelity enzyme, direct P may have no result, or the band is very weak.
Phusion is the best, the fidelity may be about 50 times that of Taq enzyme, and it is too strong, and there is basically no P.