I'll share some thoughts based on my recent experience regarding recent questions/comments:
Amex: I only have an Argentine Amex card, and I can use it basically anywhere, though cashiers are sometimes confused as I have a black card and I guess it's rare here (a few have asked if it's a special kind of card as it's the first time they've seen it). I believe all POS systems are required to take all non department store cards, i.e. Amex, Naranja, old BNA Nativa cards and Maestro debit, but cards such as CMR/Tarjeta Carrefour (both no longer issued) were subject to acceptance agreements since they were originally designed to be used in Easy/Jumbo/Carrefour and at select partner companies, not widespread like Visa/MC/Amex. I also think this is why Carrefour has moved to Mastercard in Argentina, and nobody wanted to keep the CMR brand when Falabella left (iirc Banco Columbia Argentina took over CMR and forced the members to convert their cards to Visa or MC)
For Compras Sin IVA, I think it depends on how AFIP codes the CUIT of the business. I have been able to get flat 21% refunds on items that have no IVA. For example, I had to spend 100K on medication at Farmacity and got almost 21K back (whatever the limit is) despite not buying food. I also got a refund on take away prepared food which shouldn't be eligible, but it's too convoluted to enforce which is why I believe that it depends on CUIT. The bodega next to me is principally a verduria, but he sells everything from cleaning supplies to wine and beer and as long as you pay with debit you'll get a flat 21% refund
For foreign cards, I just bought lunch and got $917.97 per dollar on my visa which is inline with about a 7% discount off of the MEP price at closing on Friday