Reno:
I would say that IF someone had a welder that came from the mfg with a cord & cap, and someone removed that and connected it with NM, MC, AC, or a raceway & THHN/THWN-2 that may raise my eyebrow. Is it any 'safer'? I would say it would depend on the skill of the person that made the change, AND...is the flexibility of the original wiring method something to be considered? Is the NM, MC, AC, THHN etc stranded or SOLID?? Things to consider!
Local (NJ) trade practice is to 'hard wire' electric water heaters & furnaces. I have not seen anyone use cord, and honestly...I have no knowledge on hand IF either can be wired with cord.
Changing a cord cap (plug) from a twistlock 120v/20 amp) to a straight blade on a pool pump will be an issue. Replacment of a cord cap on a cooking appliance with a 'like' is no issue.
Putting a 20 amp male & female, 120 volt cap on a 100' length of 12/2 MN is that an issue. Is that done? Yes! Do I like it? NO! Do I 'tag it'? Depends! Why? Basically, is it SAFE?, is it wired correctly? How long will a solid conductor survive? That is a place for cord; would I tag a 'homemade' extension cord of SO/SJ, etc.; if it was safe & wired right, NO.
