OK, here's something specific. Martinair (B742s) from SHJ eastbound A791. They are often heavy (that much we know because they're long haul to Asia with lots of fuel and cargo) When asked if they can make their requested cruising level by the boundary with Tehran, they almost always respond in the affirmative, only to find themselves several thousand feet below what Tehran is expecting them at and meaning a hurried call (read-apology) to the Tehran controller. This happens about once a shift cycle.
Now I know some of those Classics haven't got the kind of kit that some of the newer types have but please, if you have any doubt about your ability to make a level or to comply with any instruction whatsoever, please be honest enough to just say "Unable to comply" or words to that effect. I'd much rather make an extra call to co-ordinate or have the chance to make alternative separation arrangements than have to endure a near miss or do the sweating that ANSA alludes to.
For those of you who are already giving us precise and honest feedback(you know who you are), it's a pleasure doing business with you.
Safe flying!