Hi Mike and welcome!
Can't argue about a high-quality weld repair, if done properly this deffo is a worhtwile (and common) method of repairing a worn shaft.
But I strongly have to disagree with your remark on nitriding causing problems with dynamically loaded parts. Quite the opposite is true, nitiriding introduces residual compressive stress in the thin outer (hardened) layer and the bonding layer underneath, leading to an increase in endurance limit and fatigue strength.
From personal experience in professional life as well as hobbyist projects, I can only highly recommend (and have done so several times in older posts!) having a reground crank (camshaft, followers etc) nitrided. Not only will it improve fatigue behaviour of the component, but also add a very thin but hard surface layer, thus improving wear resistance and tribological properties (improved oil adhesion to microscopic pores on the surface).
It is common practice to nitride cranks, camshafts, followers, gears, valves and many many other components in automotive industry nowadays, repro camshafts very often are machined from QT (quenched and tempered) steels and the finished component is nitrided without any further previous heat treatment (quenching/surface hardening), and will give many miles of trouble-free operation.
The process temperature of nitriding is low enough not to disturb the hardened layers of either martensitic (surface heat treated) or case hardened type, and there is virtually no increase in volume or warpage involved (except on very long and thin parts such as multi-cylinder camshafts which have to be straightened afterwards, albeit only a few thou runout usually). So nitriding can be used on the readily machined component easily. But steer clear of any combined process including carburizing, such as nitro-carburizing, Tuftriding, Teniferizing and whatever the trade name is. It is too easy to disturb the delicate equlibrium of a previously case hardened part by introducing further carbon to the metal matrix!
Sorry for the long recitation, but like stated by others, you got to be careful with spreading hearsay and rumours as fact. I have no personal experience with metal spraying whatsoever, so can't and won't comment on that
Best, Markus