For starters, it doesn't work very well to call right wingers out on not being right wing enough. Because they tend to just not care about ideological consistency or honesty, you're not really getting a hook into them with this. What you are doing instead is accepting the premises they've set, even if it's just as a hypothetical. You allow them to set the agenda and to frame the conversation. That's exactly what they want, and all you're getting out of it if you succeed in pointing out their "flaws" is that they aren't as Hitlerist as they could be, which is a W for them with anybody outside of the most hardcore nazi crowd that's so unhinged they do not even care about being strategically mask-on where it is opportunate, which is a fairly small subset of their base. All their less zealous supporters can go "look, he isn't as bad as you make him out to be, you lefties just call everyone you disagree with a nazi". On an argumentative level, you're entirely playing into their hands when you come after them like that.

This also means that you forego pushback against the policy you're debating because you are only discussing its execution, not that it exists in the first place. In the case of immigration and militarizing the border, we've observed a constant rightward shift over the last couple of decades. Being anti-immigration is now a bipartisan issue in the US (and a multi-partisan one in the EU). Thousands of deaths at the border are not something that worries "moderates" any longer, being seen as weak on securing the border is. I'm not blaming that on failed rhethoric strategies, but i do see these strategies as a reaction to a previous rightward shift on the issue amongst liberal parties. They know that imperialism and climate change will lead to increasing migration, they also know that this gets racists riled up and they know that under capitalism, they will never be able to end imperialist war or mitigate human impact on the climate. So their reaction is higher fences, more drones, more internment and deportation to keep refugees out.