How can I identify which spells are incompatible with Merciful Spell?

The answers to this question indicate that the Merciful Spell feat can’t be used on Summon Monster spells, since they don’t deal damage directly. This raises the question of which other spells are also incompatible with Merciful Spell. For example:

  • Can you create a Merciful Spike Growth? Unlike Summon Monster, it mentions the damage that it inflicts in the spell description. However, the damage is still indirect, and I’m pretty confident that Merciful Create Pit doesn’t work.
  • Can you summon a Merciful Spiritual Weapon? Like Summon Monster, it isn’t doing direct damage. On the other hand, there is the Merciful weapon enchantment, and you can strike non-lethally even with a normal weapon.
  • Can you cast a Merciful Acid Arrow? It’s similar to a direct damage spell, but it’s still a conjuration (creation) effect. Also, since the acid lasts several rounds, in a sense it’s like trying to inflict nonlethal damage with an Alchemist’s Fire.
  • How about Merciful Sirocco? The damage is ongoing and arguably an environmental effect, but as an evocation spell it’s arguably more like Fireball than Create Pit.

An ideal answer would cite or suggest some kind of general rule for identifying incompatible spells, such as "Merciful Spell only works on instantaneous spells" or "Merciful Spell doesn’t work on spells that bypass spell resistance". However, answers for these specific spells would still be helpful, since they would provide at least some guidance.