Which is more annoying - a colleague who is unreliable or a colleague who is unpredictable? In my experience the unpredictable colleague is far, far worse.
School Governors are all unpaid volunteers and there is no ethical, logical, moral or practical reason why 90% of the work always seems to be done by 10% of the members. But that is the way it usually seems to be.
As Chair of Governors I am encouraged to delegate work to other governors partly as a contribution to succession planning but primarily to keep my workload manageable. The reality is that if I delegate work to one of my unreliable colleagues I can be at least 70% certain that it will not be done by the deadline. But at least if the person is consistently unreliable I know that it would be a prudent precaution to have a back-up plan for when they let the team down. I have known people who had over 9 months to do a 2 hour task but when the relevant agenda item was reached they claimed to have been "too busy" to do the work. It adds insult to injury that they usually appear totally unembarrassed by their own failure.
An unpredictable colleague - and I have one particular governor in mind - might do the allocated task well, or badly or not at all and there is no way of telling in advance which way they are going to behave. I feel obliged to "give them a chance" because at their best they are excellent but all the time the uncertainty of what will happen on the deadline day is gnawing away at me. If I do the work myself, "just in case", I might be wasting my time because when the time comes they might have done all that was asked of them. But if I don't do the work and they haven't either then the result can be very serious for the school.
Almost inevitably I do the work, feeling aggrieved and exploited, but half the time it never gets used!