From CAP413, chapter 2, page 6
I have received all your last transmission.
Note: Under no circumstances to be used in reply to a question
requiring a direct answer in the affirmative (AFFIRM) or
negative (NEGATIVE).
No mention of the last transmission being understood, only received in whole. (Although I agree with Flybymike's view.)
If you look at most of the examples in CAP413, 'Roger' is used to acknowledge receipt, before a further message is passed, with the further information or question then setting the context for the reply.
I would opine that 'Roger' alone can be ambiguous.