I'd say that theoretically, any operation on a live system involves some risk. So proper change management should account for that and should have some recovery plan in case something does fail.
Having said that - it shouldn't be that different of any other disk resize operation. Extending the disks may involve some increased I/O usage due to disk zeroing which may lead to lowered indexer performance.
Apart from that - it depends what is your disk layout - whether you have separate disk for splunk data, or just use one big device for OS and data, whether you use LVM and so on.
In general - you should be able to relatively painlessly extend the disk and the filesystem residing on it unless OS prohibits it, because - for example - you need to adjust partition sizes and the OS won't let youl So it's more of a question to your sysadmins than to splunk admin.
Edit: OK, you wrote that you're using LVM. You should be able to add some additional space to the volume pretty easily then. I don't see any big risks. Just the usual, as with any change - anything can go wrong 🙂
