I wouldn’t say the VNAV is completely unreliable.
I can say just how I use it now. It is a bit unpractical when you don’t have enough time, and then forget it, but it work
Approaching any altitude restricted point, when I am sure we will make it, I just switch to LVL CHG or VS and I simply clear the first restriction, than you again have idle descent to next point.
To PP described problem. Maybe I didn’t notice it, because in “direct to” navigation, using the procedure described above, I didn’t have such a strange indication, like completely wrong VNAV.
Happy landings.