I think that's a bit ambiguous.TT with a reliable Ground electode and a reliable RCD are as safe as an experted PME "earth" when the PME is in full working order. But hazardous when the PME is compromised and Neutral is not at ground potential.
For the benefit of other readers, do I take it that you mean that a PME 'earth' (rather than "TT with a reliable Ground electode and a reliable RCD") is "hazardous when the PME is compromised and Neutral is not at ground potential" ?
Kind Regards, John