Personally if the reason is genuinely an ATC related one I will say so, but, it can always be done in a way that is complimentary to ATC i.e. the good folks in ATC are having to restrict the rate at which aircraft get airborne so that they can guarantee everyones total safety along the route... etc. etc.
I think passengers are interested to know the why's but are not interested in hearing pilots blaming others - it smacks of "We are not to blame-itis" and raises suspicion that they may be being less than totally honest.
If all else fails blame ATC in Brussels - at least half the pax will be pleased to have another reason to bemoan the EU
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