If you need better conversion, you have to know how the system chose to handle leap seconds. If it smears, you have to know and implement the smear algorithm, if it doesn't then some unix timestamps will map to two different points in civil time (second 59 and 60 in any hour with a leap second).

For 99.9% of applications, it's not a bad tradeoff. But if we're willing to fudge time by up to a second, recording time in TAI (just count a second every second) and living with future dates sometimes being off by a couple seconds when converted to UTC isn't that different in terms of utility/usability tradeoff, and conceptually simpler.