waifu2x technology is the main part that's old I think. There's a lot of NCNN/etc. models to use now to try to tune for different details. My main problem with waifu2x is that it gives you roughly the exact same fidelity as the original picture, i.e. it's only slightly sharper than using a generic zoom function on it. Newer upscalers try to make the image more realistic, as if it was the original in the first place. In a direct comparison with the original I think there's a notable loss of detail in this method (not sure if it's inherent or just based on how the model is tuned), but without the original to compare to I think the images that the newer ones generate look more pleasing/sharp.