Restarting during an Mdadm Resync is not fatal. However, until the resync finishes, you are at the same risk of data loss you would have without the RAID getting the new disk in. Since many disk failures occur when power is cycled, there is a slightly increased chance for getting things messed up by restarting during the resync.

Disk activity during the resync slows it down considerably; this is especially evident in RAID levels other than RAID1/10 and with larger volumes.