There are concerns.
But if the TC is the only place you store these files.. it can die suddenly and is impossible to backup with another Apple product.. you cannot use TM to backup a network drive.. so you need something else.. eg superduper.. and store the files in another location.
Any suggestions for a good, cost effective external storage device that will be automaticaly backed up by Time Machine to my Time Capsule in addtion to backing up my computer. i also prefer everything wireless, but not sure if they even make an external wireless drive. Thanks in advance for any advice!
The only wireless Time Machine option supported by Apple is the Time Capsule. Wireless NAS devices will be recognized by Time Machine and will appear to work, until they don't. This will occur without warning and is therefore a poor choice for a backup strategy.
Use an external hard disk drive connected directly to your Mac, or use Time Capsule. Apple does not support any other configuration. See: Mac Basics: Time Machine
It's not as bad as it seems. Time Machine continues to maintain hourly backups on portable Macs even in the absence of its backup disk. You will lose nothing, as long as you eventually connect your backup disk.
Try it yourself. You can "Enter Time Machine" to browse recent backups even when your backup disk is disconnected. Older ones will not be available to restore but recent ones will. These "local snapshots" are written to your Time Machine backup device when it becomes available.
Time Machine is aware of local disk limitations and will not fill your hard disk with endless backups.
This is not to be confused with a complete backup, obviously a complete system restoration cannot be accomplished until you connect your backup device for instance, and if your MacBook Pro needs the space occupied by local snapshots, it will seize it without pestering you for permission.
You can also disable this option if you choose.