I love working with people like that because I can always trust them to tell me when it would be better for me to find someone else to do the thing.
That is the key quote. I'm a Hufflepuff, and hurting other people is The Worst Thing. Saying "no" isn't nice, but overpromising is much, much worse.
Some people seem to be a bit too generous for their own good. I know a precious few people who are especially good at saying "no" when asked to take on new responsibilities that would put them over their limits. I love working with people like that because I can always trust them to tell me when it would be better for me to find someone else to do the thing. I expect this to be an extremely valuable skill it would probably be good for many of us to understand, learn, and be able to teach to people who really need it.