Most of the time medical people say these things because they don’t actually understand what Huel is. Nutritionally complete powdered foods have been around for years and some of the older ones were very poor quality using things like maltodextrin as the main carb source.
There are people on the forums who are 100%. Here is one thread. I believe the only draw back was a folate deficiency. Huel has since increased the amount of folate since this thread was started.
Huel also funded two month long clinical trials of 100% users, the article is here.
A user once reported that their teeth started to feel weak being 100% Huel, so maybe using chewing gum would assist with that.