More than anything seamlessness conceals information from the
user by doing many things on the users behalf behind the scenes.
Often that is good, but not always.
Users don't know what is going on behind the scenes.
Users don't know where things are being set up for them.
When it is good it is very very good, but when it is bad it
is horrid...
When something breaks, users have no idea of where, how or why
it broke. Often the problem is with their system that "never
should have worked in the first place" but got lucky for a while.
Different things might make contradictory requests without
the user knowing about it.