A lot will depend on the materials prep, and overall quality of the joint. If done well, it is unlikely that the post will suddenly drop 1 foot. But, more likely the risk of twisting, or a drop of an inch or so. Annoying yes, but probably little more than an annoyance.
And the repair for a failed joint will be quite simple..
We have a lot of bonded materials in life, and a quality tube-in-tube bond would be one of the strongest possible bonded joints.
If the current seatpost is loose, I'd evaluate whether a larger, and perhaps longer one would be appropriate.
Get it right the first time.
Oh, and the vintage steel posts were mentioned. Not necessarily my first choice, but the twisting post toppers may have an advantage of reducing any twisting forces that get transmitted to the seatpost. Anyway, at least worth considering the implications.