There is the rendering equation by J.Kajiya and I guess every algorithm that solves it without deviating can be called unbiased.
Pathtracers, including bidirectional ones, do that by design, but slowly.
With this the "physically based" thing is covered.
Basically every accelerator introduces some kind of bias.
Like Laci said, when you don't notice it, why not?
I think this new PhotonGI version is very interesting because you can control the bias.
Noticeable bias gets introduced by caching glossy materials.
But you don't have to cache those, you can exclude them and still enjoy the speed boost from cached diffuse which has only very little bias (there's not much visible difference to reference renders as shown various times earlier here).
But when you render technical stuff with mirrors and volumes maybe PGI will not be of much use and you'll have to resort to good ol' BiDir for which LuxCore is famous.
I think it's great to have all those options!