well.
well. both the half-god and symbiont are my own creations. yes they both have limits and disadvantages because anything born can be killed.
example: the half-gods have to deal with their god-parent enemies.
also, they tend to make their own enemies at the drop of a hat (demons, devils, archdukes of hell etc). a vorpal sword can kill them by beheading (hay, even superman had vulnerabilities).
for the symbiot the disadvantages are:
you can't really die. certain neural affecting drugs can affect the neural link between symbiont and host, preventing the symbiont from protecting the host, but keeping the prisoner alive and well. If you're really clever, you can do alot of unpleasent things to said individual.
also, the process tends to kill people without renegative powers.
those who survive have major problems. like amnesia (2d10 +10 years give or take) and you have to develope a rapport with the symbiont.