In order to be sold, all seats must pass the same testing. There is no safer seat than another.
What we know about the mifold, though, is that at five years old most kids aren't ready for so little shell around them. Many five year olds aren't booster ready at all, nevermind ready for a little tiny thing under them. A harness is generally recommended for another year or so, or at least a high back booster to help transition them from a seat you've installed and adjusted for their whole lives to a booster seat that they use and adjust.
In addition, the mifold has had a huge range of where the belt lands on kids. That depends on the shape and size of the child, as well as the way the belt fits in various vehicles. For the same child it's ranged from ideal to horrible (lap belt closer to their knees) depending on what car they're in. Many kids find that it slips around as they try to get in.
Would you like harnessed or high back booster recommendations?
Wendy