Installation instructions which will say not concealed and all of that stuff, but yeah....I would be a little concerned about flow with a big appliance but....
G2422.1 (411.1) Connecting appliances. Appliances shall
be connected to the piping system by one of the following:
1. Rigid metallic pipe and fittings.
2. Corrugated stainless steel tubing (CSST) where
installed in accordance with the manufacturer’s instructions.
3. Listed and labeled appliance connectors in compliance
with ANSI Z21.24 and installed in accordance with the
manufacturer’s instructions and located entirely in the
same room as the appliance.