Is carrageenan bad for you?
Yes, carrageenan is one of the additives worth limiting. A seaweed-derived thickener in plant milks, dairy, and deli meats. EFSA's 2018 re-evaluation could not confirm it is safe — it made the daily limit temporary and asked for missing data, including on the inflammatory low-molecular-weight fraction that food-grade carrageenan contains.
Is carrageenan actually harmful?
Animal studies show gut inflammation and ulceration, and cell studies suggest it can aggravate inflammatory bowel disease. The industry defence is that only 'degraded' low-molecular-weight carrageenan does this and food-grade material is the harmless high-weight form. EFSA's problem is that nobody can currently prove that: there's no validated method to measure how much low-weight fraction is actually in food-grade carrageenan, and no toxicology on that fraction. If you have IBD, IBS, or a sensitive gut, this is the one on this list most worth avoiding.
What do regulators say?
EFSA re-evaluated it in 2018 and could not confirm its safety: it kept the group acceptable daily intake of 75 mg per kg of body weight per day only on a temporary basis, listed the missing chemistry and toxicology data, and asked for it within five years. EFSA also advised against its use in infant formula for babies from birth. The FDA still permits it for general food use, and it stays legal in the EU while the data gap is open.
Source: EFSA — re-evaluation of carrageenan (E 407) (2018) ↗So should you avoid it?
Known or suspected to cause harm: a substance EFSA or another regulator has flagged, or one restricted in some jurisdictions. Best kept out of a regular diet.
For the full reference on carrageenan, including what it is and where it turns up, see the E407 page. Or scan a product and Yuzu tells you if it's inside.