First of all, I recommend: apply for a normal visa if you're not sure what you're doing.
Basically, this e-visa is very comfortable to apply for and a really nice option if you're planning to stay only for a few days. So, what can be the problems with this visa?
- You are only allowed to enter and leave at certain points, which are listed at the official website. This also means that you're not allowed to use a flight that has a stop anywhere in the Russian Federation (e.g. Moscow).
- You are only allowed to stay a maximum of 192 hours, beginning at 0:00 AM the day you're arriving. Not from the moment you enter the Russian Federation.
- It is not possible to extend an e-visa.
- You can't leave and reenter the country with an e-visa. Once you leave, the validity of the visa ends.
- This means that you absolutly have to exit before 23:59 seven days after your arrival. If you reach the border checkpoint only seconds after midnight, you won't be allowed to leave.
- If you can't leave Leningrad Oblast on these terms, no matter the reason (delay at the border, cancelled flight, illness, delay of the flight/bus/etc..., personal miscalculation, etc...), you will not be allowed to leave the country on your own terms.
If you fail to leave on time, a five year ban from entering the Russian Federation is very likely.
I will answer how to deal with this situation in a separate question. Please add if I forgot to mention anything.