The real problem with arcane magic in RM is that while more powerful that the realm spell list, there is no significant difference in them, or their use. So the fumble range is double, big deal.
Arcane magic is raw, primal and dangerous. Add some danger to your game tagging flaws to the use of arcane such as power burn (1 hit per pp/level of the arcane spell cast), slow recovery (pp used to cast arcane magic recover at number of ranks in pp dev per sleep cycle, or half the average of In, Em and Pr mods if you prefer), mystical attraction (every arcane spell cast has a 10% chance of being detected by a mystical creature, be it demon, ethereal plane denizen, local mages, etc), magic allergys (-15 to rr's from Ess, ment and Chan), etc, etc, etc.
Without such flaws, arcane magic is indeed nothing but fancy ubur magic.