First, as to immigration, he's lost any trust on that issue given his blocking of the bipartisan bill earlier this year. He has proven that he, like every other politician, views the issue solely in terms of what it can do for him.
Second, he is far too friendly to Putin. He will basically choke off aid to Ukraine and in turn embolden Putin.
Third, he knows nothing of big picture judiciary stability and rubber stamps incredibly unqualified nominations just because guys like Steve Miller back them.
Fourth, he doesn't defend science enough. He would, but when it promotes a result the business or social conservatives don't like, he doesn't stand up to them.
In the end, it's really about the chaos he brings with him to the job. He delegates far too much to people who are not qualified, be it his kids, the power hungry evil people like Miller, press secretaries that tout alternative facts, and basically anyone who knocks on the door and tells him how great he is.
He's not a leader or an advocate for anything other than self glorification.