OP - I am not convinced that you will be able to "ignore" stainless - it will still require regular cleaning, but maybe not so critical as with carbon steel. Hence, consider what is involved to clean that bore when looking at guns to buy. I had a Thompson Centre Omega - that allowed the breech plug to be unscrewed - inspect that flash hole, then pretty much open access into the bore, from the rear. If you are looking at a rifle, may as well get the cleaning stuff to go with it, if you do not already own some - bore brush, rod and jags / patches. I was doing periodic flush with boiling water from tea kettle - then using some Thompson Center branded "wonder juice" to lube and protect from corrosion. Needed a "specific" grease on the breech plug threads. Because TC was known to have smaller than average 50 cal bore - was actually a PITA to seat sabot bullets after two or three shots - so we were carrying a patch in our mouth - spit wet - would swab the bore between shots - about the only way that the second or third sabot could be forced down that bore. I understand other brands have larger diameter bores (on average), but maybe other actions do or do not allow access to the bore for easy cleaning - is worth to check that when comparing one to another.
If you are considering a 209 primer in-line centre fire - might be worth to find out how easy to get replacement breech plugs - apparently the flash hole diameter is really critical - with use, that hole will wear and enlarge - so used to be recommended to have a spare on hand. Might not be able to get one in the future - is (or was) a reasonably expected wear / failure mode.
At the time, I discovered that various makers make their sabots in differing thickness. I think it was Harvester brand that I got from Lawry's that were the thinnest - certainly they were thinner than the yellow sabots that came with the TC Omega "kit". At that time, there was not "standard" or consensus among ML makers what size is .50 cal bore - could find them that varied 0.495" to 0.505" - yet all claimed to be "50 caliber". And that bore diameter makes a difference with a bit of fouling from a previous shot, and 0.005" or more variation among sabots.
I think it is a mistake to think to buy stainless, in order to minimize or eliminate cleaning - it will need attention, as well as the other types of barrels. You should be aware there must be a dozen or more different kinds of "stainless steel" - I think they vary with the amount of nickel content or other metals alloyed in that "stainless steel" - no doubt their ability to react (or not) to various contaminants is different to each other.
Some years ago, an acquaintance brought over a TC Omega - he had been "given it" by a friend - was going to toss it in garbage - no ramrod, bore had a LOT of build-up - many (dozens) of soaking and bore brush sessions eventually scrubbed a lot of that out - most seemed to be built up immediately in front of where the powder column might have been - and there was some broken nylon bushings in the action - I was able to make brass replacements on my lathe. I suspect that bore was a result of never having been cleaned, or at least not regularly. No doubt what residue is left behind will vary with the propellant used - Triple Seven, versus Blackhorn, versus genuine black powder, etc.