I would like to say.. deter from slim kits if at all possible. It is a fact that they do indeed run hotter, due to less insulation in the circuitry they are more "delicate". Slim is not the new style, there is a reason that they still produce "fat" ballasts.
The reasons aren't due to less insulation(that's caused by a manufacturer cutting corners), it's due to them using the case as a heatsink(when correctly built).
In correctly built ballasts, slims use the case as a heatsink thus they run warmer, but they can also fit in smaller locations. The standard sized ballast has problems fitting in skinny locations, but has better cooling due to internal heatsinks.
The thing to stay away from are chinese built ballasts altogether, as none of them design their own ballast, they only copy the designs from Taiwan(2nd best quality) and Korean(highest quality) ballasts, but utilize different circuit layouts internally for faster manufacturing and thus provide a false idea that they are their own design.
The only ballasts that will last the lifetime of a standard daily driver vehicle are Korean manufactured ballasts, and they're around $500-retail price.....you can get by with cheaper setups made in Taiwan that will last a few years on average, for around the $100 price tag(wholesale).