The down side is they are as dumb as a rock. No voltage regulation whatsoever, so you need to watch them as the battery charges. Its not unusual to see 16 volts across the clips as the charging current drops off. Lots of outgassing at that level, meaning you can decompose a lot of electrolyte in a hurry if you are not running catalytic recombiner caps.
Gennys are best when teamed with a quality charger, especially when a large bank is involved.
