I don't think they run risk of spiraling. Fav's are not shared back to the fav'rs audience, only visible as action by the OP, and in aggregate under the original message. So it doesn't serve as signal to a fav'rs own audience. Boosts don't allow remarks, just straight boosts (no 'quote-tweeting') limiting it to sharing only the original message, sharing it back to the booster's audience only. Otherwise there's only replies, which are always to the person replied to, favouring interaction. Most of all: no algo watching over what gains traction and pushing those higher up in all timelines: the timeline is strictly chronological. Meaning most of the time I do not see what people I follow boost or fav. Only in the moments I dip my toe in the river of messages do I see things pass by.