CorelDraw X5 and PDFs

I am just completing a big project that involved importing PDF ads into a Corel document. I have found that importing PDFs into CorelDraw is a hit and miss process. Some PDFs imported fine. Others were unusable or just didn't look right or had missing fonts. Fortunately, I had access to Adobe Acrobat and could export the PDFs to a bitmap file either tiff or jpeg. My questions are:

Why does CorelDraw have problems with PDFs?

Does a PDF have to be created a certain way before Corel will import it correctly?

I've tried PDF Fusion and have the same problem. If this application allowed for exporting PDFs to Tiffs or Jpegs that would solve my problem. Is this option in the works for future releases or fixes for Corel PDF Fusion? 

Thanks for any responses.