Exo incarnate + power of guardian, then what?
If you don't have enough Forbidden pieces in your graveyard, all you got on the field is just a low ATK monster that is unaffected by effect but can be destroyed by battle.
Power of guardian can protect Exo incarnate temporarily, but it also can be destroyed or banished easily.
Exodia Incarnate + Power of Guardian is not powerful like Six Samurai Shi En + Power of Guardian, since there's nothing to prevent your Power of Guardian from destruction/banishment.
Once Power of Guardian is destroyed/banished, Exodia Incarnate is just a low stats monster.
And the fact that you put Power of Guardian into your deck will makes the drawing engine less consistent.
1. Placing counters is not treated as something that directly affects the target. Therefore, the "unaffected" part does not apply, as it wasn't directly affected by anything to begin with. As a result, counters can be placed on ExoInca.
2. The effect of PotG that protects from destruction, is the effect of PotG, it has nothing to do directly with ExoInca itself.