My wife has a 390 Duke and I gave it a dose of ACF50 when she bought it nearly 2 years ago, it’s been cleaned about twice a year since then and it still looks like new so unless the 390 Adventure is built to a lower standard they should be ok.
I suppose when you compare them to the baby GS they’re not so porky but why can’t they make a genuinely light adventure bike that the rider would be happy using either on a holiday trip or for local green lanes? I’d put the 390 Adventure firmly in the new rider bracket and if there’s one way to put a new rider off green laning it’s to make the bike heavy.