Well I can't speak to any other tool's ability in terms of coding and maintenance other than Rational. I have years of experience with Robot, but very limited exposure to RFT. I wouldn't think any of them are *easy* from the start. We worked with support and along with some creative minds managed to automate the task.
I have used TestPartner, Winrunner, and QTP for mainframe/AS400 automation. in general I prefer QTP or Winrunner for this. I have seen a Robot demo a couple of years ago and it did not impress me enough to bring it in for a Proof of Concept.
Note: This does not mean it is not a good tool for this just that the vendor demo did not show it to be for us.