The short answer is don't believe most of what you read about the 6200. It was written by people who were literally just guessing who ultimately had no place doing so. In particular, don't believe literally anything written about the 6200 on Low End Mac(1).
The 6200's serial controllers are the same ones used in all the other contemporary Macs. Any given model may or may not have some firmware problems that makes the use of extremely fast modems unusable, but the best way to find out is to give it a go (there is also a testing tool, I forget which OS discs it's on). At worst, just put Ethernet in it, since LCPDS ethernet cards are easy to find. (
for example.)
(1) The context here isn't any malicious action on LEM's part, but LEM was founded in 1997, and in 1997 you could go into a used computer retailer and a Power Macintosh 6200/75 and a Power Macintosh 7200/75 were probably going to be the same price (a few hundred dollars.) the idea at the time was that they wanted to warn you against the 6200 because it is, for all intents and purposes, a worse machine than the 7200.
The gotcha here is that almost every single technical reason they put forth for why you should avoid the 6200 is basically fabricated. Someone said "oh that looks weird" and half filled in the blanks and came to an
extremely wrong conclusion.
It has been discussed at length on this forum. There are some good reads online about it, but the short version is that while the 6200 is a far cry from the 9500, it's still a competent Mac in its own right and makes sense within Apple's product range at the time.