If the reason you're going PT in the evenings is because you have a FT job, then one of the drawbacks would be limited opportunities. I can't begin to tell you how many events I've had to miss out on, from guest speakers to workshops and club meetings, because I have to work during theday... or because the event was in the evening when I had class. Similarly, most clinics require a significant weekly time commitment and don't have evening or weekend hours... by default, you need to be available during the daytime to take advantage of them. Then theres's always the restriction on study time... your FT classmates don't have the same demands on their time, so in the competition for that final grade, they have an advantage that they probably capitalized on. And then there's the summer placement opportunities... kinda hard to get actual legal experience if you're working another job FT. Of course, all these reasons only apply if you're working a FT job...
On the plus side, well... you're probably not taking out loans to support yourself, so that lowers your debt. Your employer may have some tuition assistance program that can help as well.
Generally, there are no benefits to going PT evening over FT... but if your life requires you to work to pay bills/mortgage/etc... then PT evening is an option when the alternative is not going at all.