The only time I had success with slippery elm was when I used in in the form of an enema back in the days when I still had a colon. After a colonoscopy I developed a flare (as usual). I had to take a weekend trip the next day and was looking for some quick relief. I had heard about using slippery elm as part of an enema preparation so gave it a try when I arrived at my hotel. It was a little wet and wild but I felt immediately better and had no problems the rest of that weekend.
I have not been able to repeat that level of success since - whether taking it orally or otherwise. I think it definitely can offer some symptomatic relief but probably under fairly limited circumstances.