Geometry
CB, it depends on your AoB.
At 20deg AoB at 1500ft, the error will be less than 100ft (96', I believe), probably unnoticeable over typical terrain off airport. So your RA will not "go haywire" turning final at 1500', as you say.
At 60deg AoB, the error will be 100%; ie. 50ft AGL would indicate as 100ft RA. At 75 AoB it's almost a factor of 4 - 50ftAGL = 195ftRA.
And that's assuming zero slope. If flying towards any upslope, the potential for misjudgement increases due to no look-ahead capability.
So I don't think it's a great idea to use a standard Rad Alt for low level manouevering!