Emerald guards are good, but expensive. I personally prefer Principes (atleast with Broken Empire Ermor). Fast, have javelin, still good morale, att and defence, good prot for move-2 unit, considerably cheaper...
And as Ermor, even Broken Empire, lances are only a loss of a longdead.
Emerald Guards are excellent when they can be used: that is, when speed isn't needed but you need someone tough on the frontline. Most people in this forum seem to prefer not use frontline at all, but few SCs, and some summons might be more cost effective, but Emerald Guards are very good at what they do.