Not really. I really just made them playable for testing purposes, so I did not purposefully make any changes to make starting as them more balanced.
Starting as the Infernals could potentially cause issues, the seventh time an AI tries to complete the Infernal Pact ritual. This ritual randomly chooses which of the 7 Demon Lords to summon, with any leader that has been active in the game already disqualified. If you start the game as Hyborem, then there will be only 6 choices for 7 ritual completions, which I think could cause a CtD the last time it is finished. However, the AI rarely bothers to complete the ritual at all, so it shouldn't be a problem very often. I meant to increase the AI weighting towards it, but I forgot.
Growth for the Infernals might be slower in my version, because I changed it so that their manes keep their xp from life instead of having a chance to get 2 manes from each death. (When there are 7 demon lords, having 14 manes created from a single death seemed like too much.)
Also, heroes and units of at least level 12 will be sticking around Erebus as Sluaghs waiting to be resurrected, and not coming back as either angels or manes. The Netherblade can keep units from returning as angels or manes or being resurrected, and the Malleus Vindex can do the same for evil units. The Soul Forge makes its owner's units never come back as anything, but that is a late game wonder so it would not do much to make the Mercurians or Infernals weaker sooner.