It could just be a parts issue. A lot of mechanics will work on pretty much any car, if they have the parts, but if it's not a popular model then they don't want it sitting in their shop for a week while they order parts.
I guess some mechanics will prefer to work with a smaller number of models, because they're much faster if they're familiar with the model, but new models come out every year, and they need to learn how to fix those. If a mechanic can learn to fix the newest VW, they can learn to fix the newest Renault, it just might not be worth their time if they have enough work to do.
I'm not from Poland but I hope this is hyperbolic. Parts delivery is once or twice a day delivery in most cities in Europe, no matter if it is Renault, VW or a Kawasaki motorcycle. Of course a part can have longer delivery time but not because it is a Renault instead of a Audi. At least not at any reputable delivery business in modern parts of Europe (which I would think Poland is a part of even though I haven't been there since the 90's).
That's often certain German cars in the US. IE some shops will just not work on modern Minis. Plenty of shops will avoid weird, niche cars.
Any mechanic can fix a Citroen, but is it worth the floor time it'd take to get the parts and figure out french quirks vs working on something they know that they'd make the same money in a third the time.
Having done shade tree work on various cars, I'd totally turn down any Subaru engine bay work if I was already close to swamped.
Once upon a time I was the proud owner of a third gen RX7, proud that is until the powertrain warranty ended and I had to go outside the dealership network for minor repairs. Basically your options were... go back to the dealership and pay unsubsidized warranty rates (double or triple what independent mechanic charged for "normal" engine work), or go to questionable looking characters running "performance" shops who wanted to side port everything they could get on a lift. And still pay double or triple the normal mechanic rate.
I guess some mechanics will prefer to work with a smaller number of models, because they're much faster if they're familiar with the model, but new models come out every year, and they need to learn how to fix those. If a mechanic can learn to fix the newest VW, they can learn to fix the newest Renault, it just might not be worth their time if they have enough work to do.