They are fine for recording a date or indicating a chapter number. But, they make arithmetic somewhat difficult. Here, the I's cancelled, since one of them was negative. And we also notice that years of the 21st Century will have less interesting Roman numeral representations, at least for a while. I didn't simplify any of my partial sums, because I suspected that some cancelling of some symbols was going to be possible.
Division is more difficult.
See my article, Division With Roman Numerals . This abacus, unlike the more ambiguous chinese abacus, uses four beads to represent ones, one bead to represent a five, four beads for tens, one for 50, etc. It is simple to convert from numbers represented on the abacus to Roman numerals and back again. So, in Egyptian, numbers are longer than they are in Roman numerals. But, it turns out that arithmetic is simpler.