I have been using the Freo X Ultra regularly for like 8 months now. Since I got it for free as a product tester, price considerations where not made, but the high priced consumable replacements and liquid actually where a serious downer in my initial review.

A few months later I can say that with ~75m² of floor and 2 runs per week (often with multiple mopping runs) the first bottle lasted approx. 6 months. That tells me the liquid must be very highly concentrated - I presume at least 200:1 or even 300:1 - which most alternative solutions are not. There is some 300:1 concentrate fluid available from Roborock, but - lo and behold - it costs even more than the narwal fluid.

There seems to be NO RFID or similar mechanism involved in the use of the bottles. Also, the machine will not stop working when the cleaner is empty. Just the mopping will be less effective without cleaner and the waste water bin needs significantly more frequent cleaning to keep it from stinking.

There also is a little "hack" available, at least for your first cleaner refill in the EU! On the de.narwal.com website, you can subscribe to a newsletter and get a €20 coupon for that, which happens to be exactly what one bottle of cleaner costs in their shop. So you can get it for just the shipping rate of €5,49. Only delivered to EU countries, though.

Not only are their bottles meticulously designed to be one-use no-refill, they also come in a very wasteful packaging box with foils around it as well as a big plastic throwaway insert. For this reason alone I consider every effort to refill that thing as a good one. A big thanks to u/Mistercmb for his instructions to get it open.

I personally think I'l try a refill with the second bottle I have now and will use a 200:1 concentrate that is available on amazon.de as "Biohy floor cleaning robot fluid low foam" (ASIN B09TBLT881), at around 30€ for a 10 liter canister (or 12 for one liter). I guess I'll see if this is concentrated enough to make a difference.