I heard on NPR--yes, NPR!!--that a lot of democrats disagree with the decision because a) Biden already said he would not do it, which further erodes his credibility, b) if negatively affects his legacy (which is does), and c) it removes any argument for Biden/democrats to claim any moral high ground when/if Trump pardons fill-in-the-blank.
Also of note, if the house/senate continue to investigate the crimes, which they can, a pardon strips him of his 5th amendment rights, which means he spills the beans, lies (and because of the pardon all the investigations can come out) and is charged with perjury, or doesn't show up, which is criminal contempt.