The only risk with allowing span-level spoiler markup is that the writer will be too indiscreet when choosing what sentences they wish to reveal and which to redact.
I'd like to see spoilers implemented as a span-level element: redacting (e.g. blacking out?) individual phrases or sentences, which the reader has the option to reveal.
Often a card (or paragraph in normal prose) has plenty of content that, besides carrying useful info apart from the spoiler, would also give the reader more context for use in deciding whether to reveal the spoiler.