I used to commute on slime tubes and never suffered a deflation in 11 years and nearly 40,000 miles. On one occasion I rode though a patch of hedge trimmings in the dark and picked up dozens of thorns. The tube held, and was still holding at the other end of the day when it was time to ride home. That night I took the tube out and had to positively peel it off of the inside of the tyre where it was held in place by mo less than 24 thorns. Mighty impressive.
Use them exactly as per instructions and they work well. The real downside is that they do add rotating mass just where you don't want it to can deaden the feel of the bike a touch, but in my case that was secondary to absolutely getting to work on time and getting home without issue.
If you get a massive puncture, say from a rock or pothole, then youre stuffed. No system can seal a massive split or gash. The leakage is no issue, and sluices away easily with warm soapy water - its no more faff to clean than when you periodically use the same soapy water to clean the whole bike anyway. But only an inattentive rider collects that sort of puncture, so pay attention to what you're doing and that isn't an issue either.