Trump seems to go with the first thing that pops into his head, then changes his mind based on poll numbers and what his advisors tell him. He has not a single issue in which he has taken a stand and stuck with it. There is no telling what he would actually do as president.
Yes, Trump's rapidly changing policy decisions make him unreliable. His decisions on policy and other matters of politics make him an untrustworthy individual; they make him unfit to be the next president. Even Obama said in one of his addresses that he does not support Trump as the next U.S. president.
Donald Trump has rapidly changed his major policy positions, showing himself to be unreliable. He has reversed himself on his core position of rounding up illegal immigrants. This was a major platform he ran on, and has now changed it drastically. This was done after repeatedly urging his supporters he would not back down.
Trump's rapidly changing policy decisions do make him unreliable because by constantly changing his mind, he reveals that he never had a solid plan to begin with, let alone understood all the relevant details that go into making such decisions. Trump constantly changing his policy decisions makes it hard to believe that he can actually be trusted with the presidency.
A major change like getting rid of all illegal immigrants has various aspects that even Trump may not have considered. I am sure he realized that it is something that could not be done overnight. Thing is, to begin the such a change, first he has to be elected. This means he must convince the public that the change will happen rapidly even tho it may take much longer and may require some compromises.
A clear example is Lincoln's policy against slavery. Because of the vast amount of slaves and how many state economies depended on them, it was much longer process than the public thought it was even after the war.
In the case of illegal immigrants, clearly we need to deport those that commit serious crimes first. Seeing that many agricultural industries still require cheap labor, we would need to find a way to ween them off of illegals. One step would be to encourage industry to design and create harvesting machinery to replace them just as slavery was replaced by machinery in the south.
Imagine if Lincoln was questioned on how realistic his anti-slavery position was, seeing that it would still be needed in some form until replacements could be made available. Odds are he would also state that it could take quite a while and multiple steps. Tho it may seem like Lincoln may have retracted his initial stance that slavery must end now, it isn't as the goal is still the same.
Another thing is, Trump is also a businessman. Business requires some give and take so it's no wonder that he may back off some issues in order to achieve the greater goal.