Just to add another data point here, partner and myself both rejected by AMEX. Both applied for the AMEX Velocity card to take advantage of the 30k bonus points during April. I've churned 3 cards in the last 12 months, she's done 2. Both on decent income, good credit score and repayment history, paying balances in full etc.
She's had an AMEX 5+ years ago and I've never had an AMEX but clearly the churning was a factor. Applied for Virgin Velocity High Flyer and was approved on the same day I found out I was rejected by AMEX