When you say second year transfer, do you mean entering as a rising sophomore or a rising junior? If rising junior the upside is that your acute senioritis from HS won’t be an issue- and the downside is that both UChic & Columbia are less attractive (b/c of the challenges of getting the Core done).
If rising sophomore, it’s hard to say.- it may partly depend on just how bad the final senior grades are. AOs know that senioritis is real, and they may overlook some amount of it- but they will only have 1 semester of college grades, and who can know if they will?
And that brings us to your question: “how hard is it to be accepted as a transfer?”…well you could have answered that question for yourself: transfer acceptance rates are Yale, 1.7%, UPenn, 7.8%, NU 15%, Columbia 6%, and UChic 5%. Cornell’s rate is complicated, b/c they have several guaranteed transfer programs.
Since you haven’t given particular reason for transferring, I’m inferring that the reason is that you didn’t get your first 6 choices and your ego is bruised. Other people have successfully transferred for that reason- but most of those schools are well used to students who are taking a second run at the prestige ring, and they will be looking for a genuine reason that you should join their community.