I used a similar service called lifesum (also not foss) a few years ago to successfully lose 10kg. I used the free version.
However at the start I briefly tired both but decided against yazio because the data they asked for was extremely intrusive.

First thing to note is that all of these apps overestimate the calories you should eat by a lot.
They told me I could eat 1700 calories per day to lose weight but I was eating about 1600 calories before and gained weight.
I went to a doctor to get my more accurate calorie requirements which was 1400 calories to maintain weight and 1200 to lose weight (I have an illness that affects my metabolism). I’d recommend doing the same if you can.

Next thing is that the catalogue they use for items is filled by people and thus can be inaccurate. I recommend scanning the barcode and then still manually calculate the values and comparing both for every new product. If it is off you can make your own entry.

For products that don’t have barcodes I usually pick the ones that are exaggerating the nutrients. Like one listing for egg will say 60 and the other will say 90, I’ll pick the 90 one.

Also keep in mind some products have different values for cooked and uncooked, like rice, be sure to pick the one you weighted.

You’ll only need the app for a little while since you’ll develop the ability to calculate the calories effectively enough in your head after a bit.