Red P+HI reduction [duplicate]

My teacher told us that Red P and HI can reduce any functional group. Is it true? Or are there any groups it can't reduce?(I know this exact question has been asked before but I couldn't find the exact thing I wanted)

I need a list of functional groups which cannot be reduced but red P HI

• One group that P/HI won't reduce is -COOH. Kiliani reduced glucose to hexanoic acid with this reagent. – user55119 Jun 17 at 13:22
• The linked question answers your question to the point. What could you not find there? – Aniruddha Deb Jun 17 at 13:23
• @AniruddhaDeb the ans referred is scattered. I couldn't find many results from that – DatBoi Jun 17 at 13:26
• To OP:See this. To @user55119: carboxylic acid can be reduced (for ex-benzoic acid reduced to toluene and another source saying lauric acid can be reduced) – Nilay Ghosh Jun 17 at 15:02
• @Nilay Ghosh: Aromatic acids may be an exception. As to lauric acid, need it carefully. The commentary is shear speculation. Not a reference in sight. – user55119 Jun 17 at 15:14

In short, the first substance contained the $$\ce{-OH}$$ and $$\ce{-NHCH3}$$ groups, and the reduction took away the former, but not the latter. The synthesis could be done with surprisingly crude equipment, so the junkies did it every time they needed the product. They bought red phosphorus and crystalline iodine by kilograms, and carried them around in a form of pre-prepared mixture ready to go into the reaction. They used to call this mixture "Stendhal", because of "The Red and the Black".