The way I see it, the extra card boost is, in the most practical sense, mostly for soloers and Summer BB locks. However the refund breakpoint changes will definitely help certain ST servants reach that all-important 49% refund from a chain. A couple of my favorite bronze and silvers sometimes land around 13%-2% short of a breakpoint (e.g… 50%, 80%, or 100%) from an NP overkill chain, so the extra card boost will enable them to loop bosses, or reduce the amount of times they need to be attacked by the boss to refund up to 50%, effectively reducing RNG. But I don’t think most players depend on (or care about) that kind of thing. Most people stick to guaranteed strength over situational buffs.
Example users in a (for lack of a better word) meta sense include servants like F/SN Cu, Heracles, OG Nero, Asako, Barghest, etc… People who you want to take up a whole deck already. That’s what most people will recommend, because they’re usually using their extra card every turn.
More interestingly, I think the servants who benefit most from this in a general sense (looking at each servant in isolation) are the servants with dreadful hit counts. A lot of decks are full of 1-2 hit QABs and then a 3+ extra, so at a purely base level their extra cards are their best, mostly to reward how unreliable the Extra Attack is to access.
So it’s really a “you love this servant, so here’s a small boost” type of skill. Though if you love a servant that much, you’ll probably have used them enough to know whether the skill benefits them much, whether you use their extra card often enough, or if the boost will even help you reach a goal you’ve previously fallen short of.