I have highload on writes database. It's safe to copying files as backup mechanism, or better use export api?
Reading the documentation
Backup does a consistent copy of database, all further write operations are locked waiting to finish it. The database is in read-only mode during backup operation. If you need an read/write database during backup setup a distributed cluster of nodes.
Export, instead, doesn't lock the database and allow concurrent writes during the export process. This means the exported database could have changes executed during the export.
I think that if you copy the folder of your database and your database have some problem that problem remains while with import and export it could be resolved.