How serious is SVT if someone had chemical cardioversion?

Not usually serious. That type of rhythm problem is usually more of a nusance than serious. It can at times be hard to treat as far as preventing recurrences but except for the nusance aspect is usually pretty benign. It would be helpful to know the exact mechaism of the rhythm problem and the heart rate in answering your question.
Not related. Cardioversion, either by chemicals or electricity, terminates that episode of svt. It can recur. The danger is not related to the cardioversion. It's related to the rate of the rhythm and the general health of the patient. Some types of svt are well tolerated and benign. Some aren't. One cannot generalize.