I won't name the company out of respect but its a big one in the UK, it was the live customer service that told me about the stability issues, I might be best contacting them in an email.
Say I do take the lesser speed ram, is it a major deal on the system as a whole or are we talking maybe 10 or 20 less fps ?
Doesn't matter
who it is, although smaller companies are probably a bit more up-to-date vs support staff handling live chats.
As logain mentioned, 1st and 2nd Gen Ryzen was pretty problematic >2933MHz so, even if they were harking back to their previous experience with 1st & 2nd, a 3200MHz kit doesn't really hold much weight as an argument either.
FWIW, that board has 4266MHz kits verified as working & appearing on the QVL.... alongside countless 3600MHz kits.
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I wouldn't have any issues if a builder was only prepared to provide kits from the QVL, as they'll be warranting the system (right?!) and using only verified kits gives one less thing to worry about. That's not what they're doing.
We're not talking the difference between 'playable' vs 'unplayable', or anything remotely close....
Example (lots of articles out there)
https://www.gamersnexus.net/guides/3508-ryzen-3000-memory-benchmark-best-ram-fclk-uclock-mclock
....BUT if there's performance to be had, and you're already forking out for a £500+ CPU and £600+ GPU, don't you want to squeeze as much performance out of it as possible? Especially for the sake of £5-£10 retail price difference between a 2x8GB CL16 @ 3200MHz and 2x8GB CL16 @ 3600MHz?