The reason DNA is useful as criminal evidence is you leave it everywhere; that makes it a horrible security factor: like fingerprints it's an impractical to replace wheb compromised “thing you have” that you leave everywhere for attackers.

That is a very good reason to not use it alone. However when combined with other it provides a good indication that someone was actually there. Either that, or the attacker went through more effort to get the secrets.

Maybe? Although not yet practical, in theory it too can be captured and reproduced.