I disagree. The 1.5 % is not an interest rate, it’s a fee that is paid once on charging.
Basically, if you save more than you pay, it’s always still worth it.
Cost: 1.5 % - 0.3 % cashback as Cumulus (assuming you treat it like money) - 1.2 %
Benefit: Currency savings by using Revolut - 2-4 % depeding on your card.
With this back of the envelope calculation you’d always still be better off to pay with Revolut instead of with Cumulus by 0.8%. On 1000 CHF this is till 8 CHF.
Of course it’s better to avoid the 1.5 %, but you still save money all things considered.
I have made a comparison of Swiss Credit Cards and their benefits and costs:
Charging methods for Revolut - Costs and benefits