Vance,
I can only speak from my personal experience. A friend had a Cimarron '73 in .44 Special that he shot with .44 Colt. After spending a day shooting it, I decided I wanted a '66 for the same purpose. I got a '66 carbine and never had a problem with it, except for the loading gate breaking.
I liked the carbine but had come to enjoy shooting the rifle length lever actions, so I bought a '66 rifle in .44 Special and sold the carbine. Again, never any problems. BTW, the man who bought the carbine had it modified to shoot .44 Russian and says it works great.
A former Cimarron 'smith told me that sometimes the extractor has to be slightly modified to grab the smaller rim of the .44 Colt, but since I've never had any problems I've never had to try that solution.
As for blowback, I put a really tight crimp on my BP cartridges and have yet to have any significant blowback. In fact, I've had folks ask me what kind of BP I was shooting that I didn't get any fouling in the action. I do the same with my .45s and have the same results.
I've had good experience, but I also realize that sometimes firearms have their own personal gremlins and it there are some .44 Special rifles that will not feed the .44 Colt. The Cimarron 'smith you talked to is giving your the required line to avoid any liability - if you shoot a cartridge that the rifle is not designed for, they are not responsible.
As for the Spencer, I have one of those, too. My first one was very sensitive to OAL and tended to jam more often than feed the cartridge. Taylor's eventually replaced it. My new one is great. They are, however, extremely slow to shoot in comparison to the Winchesters and Marlins. That doesn't bother me as I'm already slow and don't care about my time - just having a good time. Besides, the Spencer is great for style points, especially when shooting it in Frontiersman.