Castle trust bonds: avoid? - NOT amateur radio

The top of my Gmail seems to have regular ads for Castle Trust offering bonds with very good interest rates.

I read somewhere that these funds are "dodgy" and you could end up losing capital. I have no idea whether true or not, but personally I shall be avoiding them. I once read that if something appears to be too good to be true then it is and is best avoided.

Generally, interest rates are poor.