IMO JBF was giving her best and was involved in her role till the end, unlike Dan. Dan actually contributed to making Matthew look quite pathetic in S3. What we actually saw in S3 from him was non-acting. I swear if there were equivalents of Golden Raspberries in TV world, he would surely get a nomination, if not a win . IMO it's simply the more and more critical atmosphere around DA that contributes to negative opinions even the ones concerning the actors.

Because of DS, M/M also lost their chemistry, whereas S/T preserved their incredible chemistry till the very end.

Frankly, when I look at Matthew I think that he actually regressed - he's never been a particularly interesting character, but IMO in the end he regressed to a randy sap who was complying with Mary's every wish (though JF turned him into such because of his impending demise). Sybil at least clearly matured throughout 3 series. The problem with her in S3 was not that her scenes were bad, it was that there wasn't enough of them and a lot of things that should have been shown - weren't shown. And of course her death practically destroyed, wasted and undermined eveything what was before, but we've all already elaborated on this here multiple times since 3x05.