One of the thing I'm never going to forgive the left for are the times I've had to defend Trump, who I dislike.
Announcing mass arrests that aren't going to occur might be ethically flawed but see below), but there is nothing wrong with mass arrests of those who have broken the law.
And it is at least arguable that Trump's mass arrests announcement finally panickedthe House Dems into agreeing to a bill to adequately fund detention centers. (Most House Dems, like most liberals don't actually want an unguarded border with people pouring across and camping out in the liberals' neighborhoods, but for some reason they can't actually come out and say that because, again for some crazed reason, it is politically incorrect for them to do so).
As for the left (as opposed to the liberals) their rhetoric is off the charts. Concentration camps? Genocide? Attacking ICE agents?
The crisis with the immigration system in the US today is largely the product of a deliberate decision by various leftist groups to encourage large numbers of Central Americans to come to the US and (falsely) claim asylum. Their purpose appears to be that they want to change the US' voting demographics--and right now they are succeeding. Trump didn't do that and while his rhetoric might at times be off-putting, his instinct that the US needs to defend its borders is widely popular and I think correct.