First-order tets, explicit or otherwise, are still constant-strain elements and will yield overly-high stiffnesses (and result in poor results) and poor convergence.
Second-order elements in contact can yield spurious contact stresses in many situations. This has to do with the implementation of contact algorithms as applied to 2nd order element formulation. In this case, contact-intensive problems (which often happen in explicit analyses) could yield questionable local contact stresses for second-order elements (although far-field responses should still be reasonable).
First order tets, in contract, may give apparently "cleaner" contact stresses, but the underlying formulational problems still exist, and I would not trust those first-order tetrahedral results.
Some codes have specially-formulated 2nd order elements which address the 2nd order contact issues. In this case, these elements provide not only superior element response but also reasonable contact response.
In short, for explicit non-contact problems, 2nd tets are definitively superior, but for explicit contact problems they may have problems at the contact patch (which is addressed in some codes).
Brad
