Unreliable Fight Styles
If you see an "Unreliable fight style!" warning on your sim, it's because your class/spec is not optimized for that fight style.
This usually means the APL has not been tuned for that specific fight style and/or there could be obstacles for that spec to properly support the fight style.
If you see this warning, your best bet is to consult your class Discord, search for previous conversations about the fight style, and then see if alternative are suggested.
Dungeon Slice is the probably the most common fight style where you will see this warning. Many specs prefer using a multiple target Patchwerk sim rather than Dungeon Slice. Some prefer a more recent and experimental feature called Dungeon Route which can be used in Expert Mode. Suggestions will definitely vary between the different class/spec communities and you should seek more specific info there.