LMOL because digital signatures are secure *eye roll* - you should be well aware that nothing digital is secure.
Nothing is 100% secure. Some things are more secure than others.
Pen and ink signatures can be copied by a fraudster with sufficient skill at least to a level where a casual person would not notice. In many cases only the last page of a document is signed so earlier pages could easilly be substituted by a fraudster. I'm sure I also heard of attacks where a page was erased after being signed though I can't seem to find any references right now. There is also the issue of finding a good reference to compare the signature against.
Photocopies, faxes, scans etc of pen and ink signatures are worse. With a real peice of paper it's pretty hard to cut out the signature and paste it onto a different bit of paper without getting noticed. With copying involved it's much easier. A fraudster can get a clean copy of the signature (may involve a little bit of work taking a bad copy and cleaning it up but no particular skill involved), and paste it into a clean document. Since the document and signature are both clean there will be no visible "seam" to indicate the pasting. If grot is considered desirable to hide the digital origins of the document it can be added after the signature is pasted in by printing and rescanning.
I've seen at least one case where someone "signed" a document by pasting their signature into a word document and sending said word document to me. Doing the same thing with pdfs is even more common.
Cryptographic signatures have issues of their own such as verifying that the private key actually belongs to the right person, the risk of the private key being stolen, the risk of poorly generated or inadequate length keys being factored and so-on but compared to scaned/faxed/photocopied copies of paper signatures I would say they are far more secure.