DEH had its premiere at the Opening Night Gala at the Toronto International Film Festival, which is one of the major film festivals and is often a launching pad for Oscar-aiming films. Green Book and Three Billboards, for example, both launched there. That DEH is not yet in release is why there aren't that many reviews, but the major critics are all there and are chiming in now. So their mixed response has some meaning, although if the film outperforms at the box office, that mixed response will have less impact.
I noticed that Ben Platt has a full beard and mustache for the film promotional circuit. Not sure that is going to help the concerns that he's too old for the part.