While I'm not sure how this would be similar to MSG, the charges against carageenan are (a) it causes intestinal ulcers and/leading to gastrointestinal cancers, and (b) it causes intestinal inflammation, leading to irritable bowel disease.
(a) is based on a number of non-human studies that were fed large amounts of *degraded* carageenan, or poligeenan, which is not used as an additive (undegraded carageenan is used) and is a possible carcinogen. The theory is that either degraded carageenan is in the additives, or it is degraded during digestion. As for the second, there is not sufficient acidity or heat in the human body to degrade carageenan. For the first,
A recent publication (Uno et al., 2001), which examined 29 samples of food-grade carrageenan, showed that the weight average molecular weight of these samples ranged from 453-652 kDa and that poligeenan (20-30kDa) was not detectable in any sample.
So, it is extremely unlikely that poligeenan is present in sufficient dosages to be carcinogenic, if at all.
(b) Is based upon the study here:
Undegraded carageenan's effects were tested on rat cells, human intestinal cells, and human colon cells. I'm not a rat, so I'll focus on the other two. Intestinal cells showed increased markers for inflammation over controls as early as two hours; colon cells took 24. So, there is a statistically valid correlation between non-degraded CGN exposure and inflammation. A few things on this:
1. It takes 3-4 hours for food to pass through the small intestine, and 30-40 to get through the colon. Considering that the times to inflammation for each were towards the end of time food would normally spend in each organ, it's possible that inflammation is a normal response to the end of a phase of digestion. A second control, exposed to something widely considered safe, or even better, a variety of controls, would sort this out. (Note: I am not all-knowing; this may be the most ridiculous assertion ever made. Biologists feel free to let me know if this is impossible.)
2. CGN is a polysaccharide; that is, a carbohydrate. Considering the website I'm on, I doubt I could be faulted for hypothesizing that ANY carbohydrate would irritate the colon. IMO, more controls are needed for this to prove that CGN singularly and specifically, not just CGN as one in a large category of molecules, is inflammatory.
TL;DR: It's likely no more harmful than any carbohydrate, but since it's not necessary for anything, avoiding it wouldn't hurt if it bothers you.