H.264 has the following disadvantages when using as an editing format:
1. H.264 is mathematically intense. It takes some serious computer horsepower to decode its compression.
2. Because it is so mathematically challenging, it takes longer to render H.264 files than other formats.
3. As H.264 is an 8-bit format shot by HDSLR, which means you are potentially compromising your effects and especially color correction and composing gradients.
4. H.264 does not integrate easily with other video formats.
Therefore, it would be ideal for you to choose Premiere Pro natively optimized MPEG-2 file format.