There's an interesting legal conundrum that Mitch McConnell is in the center of. Trump abused the acting head policy by never actually submitting anyone to McConnell because the majority leader would then have to hold hearings on the nominees (which would have been bad political press, because the nominees were NOT in general competent for their job). Biden will be submitting his candidates to McConnell, who then MUST hold hearings, or the candidate in question will automatically be appointed after 90 days (I believe). McConnell will have at best a razor-thin majority in the Senate (if only one of the Democrats wins) or will lose control of the Senate (if both win). He's also going into an election cycle in 2022 where the Republicans are vulnerable, and this doesn't rule out a defector in the Senate (my guess would be Murkowski) switching to the Democratic side. This is going to make it a lot harder for him to obstruct appointments.
From the legal side, because Trump failed to submit candidates to the Senate for review, he is effectively in violation of the constitution in his appointments. Biden WILL be submitting candidates, so this won't be an issue. It also means that a significant portion of the EOs that Trump wrote will be found non-binding precisely because his appointments were not reviewed.