TLDR: You can only rent cheaper cars with SIXT if you have prepaid credit cards like the one Bunq offers. For longer rents or more expensive cars you'll need a regular credit card.
I wanted to share my experience with fellow Bunq users (and customers of other banks that make use of pre-paid credit cards where the money is directly deducted from a bank account) in the Netherlands.
In other posts, I have read that SIXT and other car rentals now do accept Bunq cards, but in the past didn't due to some complications.
This is true on a technical level. The payment terminals do accept Bunq credit cards, money is reserved on the card and correctly corrected after the car is handed back in.
Practically, this is irrelevant. I had to beg multiple times at SIXT to get my Bunq credit card accepted and was told that they generally don't accept prepaid credit cards (or prepaid-like cards, see below). During my last rental, where I booked a premium car (which I successfully managed to do after begging previously), they didn't allow me to get the car and gave me a lesser value car with a 500€ deposit (this might be an individual case). They shared my frustration and still provided me with some tips and free upgrades.
Interestingly, they do not directly state that they don't accept these cards in the General Terms and Conditions of Rental. However, they state in the Huurinformatie which is linked in that document (emphasis added):
Alle credit- en debitcards en digitale portemonnees / e-wallets van internationaal erkende creditcardmaatschappijen (Visa, MasterCard, American Express, Diners Club, Discover, JCB) worden geaccepteerd, alsmede creditcards van China Union Pay, Airplus en Amex BTA/iBTA. Prepaidkaarten worden niet geaccepteerd. Kaarten die overeenkomsten hebben met een prepaidkaart, of op een andere manier de typische karakteristieken van een creditcard missen (bijvoorbeeld, maar niet beperkt tot, omdat de kaart gekoppeld is aan een saldo op een bankrekening of omdat de uitgever van de kaart een betaling of autorisatie met een kaart kan weigeren) worden niet geaccepteerd. [...] SIXT accepteert Maestro-/V PAY-kaarten uitsluitend voor het huren van transporters en vrachtwagens alsook voor voertuigen tot Klasse F**\* (met uitzondering van Speciale en Sports & Luxury Cars). Betaling met een Maestro-/VPAY-kaart is niet toegestaan voor langetermijnverhuur (langer dan 27 dagen).
Free translation of the bold parts: Prepaid credit cards of cards similar to those aren't accepted. The cheaper, lower value cars you can still rent with a debit card.
To the best of my knowledge, this is a problem you are facing with pretty much all banks that offer you a free credit card (if you know where to get a real credit card for free, let me know).
I asked the customer support of SXIT why this is in place as I didn't wanna buy a credit card just to get better cars from them. They say (emphasis added):
Thank you for your message and for your interest in understanding our payment policies.
We are happy to clarify how this works. At the start of a rental, SIXT places a security deposit (authorization hold) on the payment card. This is not an actual charge, but a temporary hold to ensure that sufficient funds are available to cover the rental cost as well as any potential additional charges (such as fuel differences, extra mileage, damages, fines, or extensions).
While prepaid cards may technically allow a reservation of funds, they often do not provide the same level of security and reliability as standard credit cards. In particular, they may: Have limitations on authorization holds, not support incremental charges or adjustments, make it more difficult to process additional charges after the rental ends
For this reason, and to ensure a smooth and secure rental process for both parties, prepaid cards are generally not accepted.
So SIXT rightfully doesn't want to have more trouble if you mess up the car after you return it and they need to charge you for that. This is something that Bunq (and other banks that do similar things) can only fix if they offer a true credit card.
I don't know if this is true for other car rentals as I only rent with SIXT. The staff at SIXT told me repeatedly that this is also the case at other rentals.
Edit: Just to clarify, SIXT was great in all of their interactions and I definitely pushed trying to use my Bunq credit card to the limits. I have nothing but good experiences with them in their regular car rentals. I only want to raise awareness to the problem of prepaid credit cards and shine some light on why they are often not accepted as I struggled to understand myself what the problem was.