You are limited as to how much information you can keep in the "spoiler" as the display box will only be so wide.
Correct. Though I'm not sure what good it would do to contain large fields of text in a spoiler box. You're not exactly hiding detailed national secrets. Hopefully whoever is structuring the reply will make hidden text a reasonable length.
Also, if someone is not sure what you are doing, they may spend an inordinate amount of time trying to figure out what is "behind" the text
"█████" without success, and never actually see the spoiler text.
Replaceable with any alternative text, like [hover] or [spoiler]. I figured the conventional "black box spoiler" was clear enough but again, audience targeting is at the discretion of the post-bearer.