Why the industry moved
R-134a has a global warming potential of 1430. R-1234yf sits at 4. Regulatory pressure — not performance — drove the swap. Cooling performance is nearly identical, but the chemistry is mildly flammable (A2L classification) and the service equipment has to actively refuse to work on a contaminated system.
Service equipment differences
A yf machine will not recover unknown refrigerant. It runs a refrigerant identifier before it opens any valves, and it locks itself if it detects more than 2% contamination. That is not the machine being difficult — that is federal law protecting the tech and the customer.
Common failure modes
1234yf systems are more sensitive to moisture and non-condensable gasses than 134a. Any repair that opens the system requires a full evacuation to 500 microns held for at least 15 minutes, and a replacement receiver-drier is not optional. Cutting corners here shows up as intermittent cooling within a season.