Compatibility with the IBM 360:
The topic of the Univac 90/60's compatibility with the IBM 360 is something that actually provokes a bit of controversy when discussing the history of this otherwise rock-solid computer system. Some will argue that it was effectively a "clone" and others will tell you that the supposed "IBM compatibility" was a myth promoted by the marketing department. My own opinion is that the 90/60 was nominally compatible with the un-privileged instruction set of an "ideal" but non-existant computer somewhere in the middle-range of the IBM 360 family.
From the perspective of a computer science student writing student-level programs in IBM's "Basic Assembler Language", you could easily write an assembly-language program that would run on both the IBM 360 and the Univac 90/60. From the perspective of an application programmer, writing business applications on the IBM 360 was much different than writing those same applications on the Univac because the I/O was handled differently and the JCL (job control language) was MUCH different. I was never a systems programmer on either machine, so I don't have much to offer in that regard.
I can tell you that the system libraries and MACRO instructions were different enough that reasonably complex programs written for one system would often bomb on the other. Likewise, the differences in the "user experience" made the systems feel very different. I preferred the Univac to contemporary IBM mainframes... Mostly since the JCL and error messages on the Univac made sense to me!