The general consensus is they do offer a nice peace of mind and protection against flats.
Some of the downsides include increased weight and can be a bit messy.
Sources:
https://www.pinch-flat.com/slime-inner-tube-review/
https://www.amazon.com/product-reviews/B000ENMLFK
https://singletrackworld.com/forum/topic/what-are-your-views-on-slime-and-similar-inner-tubes/