I badly cracked the head tube on my run-of-the-mill steel frame (Reynolds 501 CrMo) and a guy recommended by a bike shop welded it up for me for £25. Six years later it's still going strong.
And I've taken the bike off 6ft drops and ridden it decently hard most of it's restored life.
CF repair can be done and plenty strong, it's just more risky because the properties aren't isotropic and simple like steel/alu so you can't just weld in a piece of metal of roughly the same thickness. I would not want to repair a bike I don't ride myself.
[1] https://medium.com/@fixingthings/fixing-a-trek-remedy-29-9-8... (yes, sorry it's medium)
Welding after heat treatment will reduce the weld affected zone back to its basic untreated properties.