There is not a size limit. Lot of these other methods actually use GNU Tar behind the scenes anyway. More then that GNU tar has been used for decades for this purpose. Pull out any Unix book from 2 decades ago and you will see "tar", "cpio", and "dump/restore" as the way. The new tool out there is Pax and in fact GNU Tar supports the new "pax" format. Moreover GNU Tar with Pax format can backup almost full disk structure including hard links, ACLs, and extended attributes which a lot of tools do not do. It is still useful to archive some things at a lower level like your partition table, and boot blocks of course. You also have to decide what run-level (such as rescue) you want to archive in, and/or what services you should stop, or provide separate to file system dumps for depending on your system. Databases, and things like ecryptfs take some special thought (thought it does for any tool). It is also good to do test restores to verify your disaster plan.
I use tar on many systems. My workstation is about 1TB of data. Backup is about 11 hours though I think it could be faster if I disabled compression (I currently use the standard gzip compression which is not optimal). I think the process is CPU bound by the compression at the moment. Going to uncompressed or using parallel gzip at level 2 is probably the fastest you can do and should really speed things up by 4X or more. I have played with this some for my wife and her raw backup is a lot faster now. My wife uses USB 3 external drives specifically plugged into USB 3 ports (the one with the SS symbol and the blue interior), and with a USB 3 related cable. I use 6TB naked SATA drives I insert into a hot mount enclosure and store in storage boxes. My backup system can theoretically do incrementals too, but it has some issues since I have moved to BTRFS so I do not use that at the moment. Did always use before. I have an idea how to fix, but need to debug and test incrementals now.
How often: I backup monthly. When my incrementals were working I use to do it weekly or whenever I got nervous. Other option for the BTRFS file systems would be to use their native backup tools. Not sure though, I like to use generic stuff. Lot to be said for generic.
Big downside of tar is the mind numbing man page. Getting the options correct takes some real thought. You also have to be comfortable with the shell and Bash scripting. Big upside you can customize exactly what you want.