Most projects start with the planning phase by detailing the assumptions on which the plan will be based. Some of these assumptions are estimating guidelines, as in "50% of the project resources will be subcontractors". Others are scope limitations, as in "the client will be responsible for all data conversions". Each of them is a potential risk, but in a different way. And the level of risk is not comparable.
Planning guidelines are clearly understood as being directional and subject to constant review - if you cannot get 50% subcontractors, or more likely 50% of regular employees, then the team leaders will quickly let you know. Scope limitations are exclusionary actions - stuff is being discarded and will not be considered again. This may not be subject to constant review and out of sight means out of mind. So the risk engendered by exclusionary assumptions can be much larger because it will come as a surprise if it is wrong or changes.
The example quoted, "client will be responsible for all conversions" is a particularly dangerous one in that we have delegated responsibility for a key element and may have surrender key oversight as well.
Scope creep is always held up as a persistent problem that we need to be on our guard against. Yet in many cases it is unwise scope exclusion at the planning stage that causes us more serious problems. Scope creep can be a slow process with many opportunities to halt it. Like an increasing waist line it may be insidious but there are usually plenty of signs that it is occurring, like having to take your belt out a notch or two, or getting a larger dress size.
With scope exclusion assumptions there are no early warnings until it all goes wrong.
What you expect to happen doesn't and you are in trouble and everything is at risk. It's the difference between obesity and a heart attack. You can always diet but you might not survive a cardiac arrest!