You will get both opinions on the P-51.
I personally think the Hanger 9 P-51 PTS trainer is not a good beginner airplane and yes I do have one and have been flying it.
Take almost any high wing trainer and you will get almost unanimous approval, not so with the P-51.
Bottom line, some people learn with the P-51 and some people find it too hard to handle and move on to a conventional high wing trainer where they have better success.
The good news? If you do buy the P-51 and find you can't handle it you can transfer the engine and electronics to a high wing ARF trainer at a fairly low cost and still have the P-51 (if it is still in one piece) for future use.
My opinion? The H9 P-51 is a good second plane.