Loaded question Boon.
It is really going to depend upon the end use strength required, any codes involved, any jurisdictional involvement from a gov't agency, type of support, etc.
Under many circumstances it will work. It just depends upon what they want and/or need.
There are many ways they could compensate for the diameter difference, including leaving it and making sure there is a good transition so thickness is not an issue.
The gap can be ground out for more clearance in the tight spots. 5mm should not be too wide.
Safety is always more of a concern than some of the other factors. But with the right welder on the job it would probably be fine.
So, any codes or jurisdictional issues?
Have a Great Day, Brent