I don't know that it's *never* appropriate to put NM cable (Romex) in conduit. Type UF cable can go through conduit for short runs, such as under a concrete walkway.
There are several reasons, though, why it is not desirable:
1. Might exceed conduit fill resulting in heat dissipation problems. You might have to use larger conduit than would otherwise be necessary.
2. Possibility of damaging/kinking the Romex, especially at the 90 degree bends.
3. Waste of money. Use individual THHN/THWN conductors in the conduit instead.
I don't recall anything in the code prohibiting Romex in conduit, but I add this disclaimer: I am not a licensed electrician. Wait until one of them answers to be sure!

In my neck of the woods NM is not allowed in any conduit that is installed in what is considered to be a wet location. That includes EMT and PVC conduit. Putting NM in conduit in a dry location, for physical protection only, is fine.

"I don't recall anything in the code prohibiting Romex in conduit..."

A few code revs ago the wording in the NEC was altered to indicate the individual articles would need to allow the wiring method to be used in conduit.

Many of the separate articles have not been updated with the permission for general use in conduit.

NM is allowed in conduit for physical protection, but that is not the same as a 'conduit system.'

If by a "screw on connector with a 90 degree" you mean a tight 90 degree conduit fitting, you will not have adequate bend radius unless you use larger conduit.
You also cannot strip the NM jacket off to reduce the size since the individual conductors are not marked for type.

The solid conductors also make it a PITA to pull.

The big box stores sell cut lengths of THHN stranded wire.
Get black, white, and green and pull them into the conduit.

I guess the blue Carlon corrugated conduit has the same rules. When my basement was finished, I installed Carlon conduit from the main breaker (furnace room) across the room to my laundry room that has a drop ceiling. The rest of the basement has drywall which would have future access issues. I since ran a 3 wire cable to the laundry room inside the Carlon conduit.