No TB drives are fine as OS drives. Just make sure you have backups. And remember the fact that any drive from any manufacturer can die at any time. Failures are not always caused by usage or old age. At work we have a 1% to 7% annual failure rate of all drives. And many of these failures are less than 2 year old drives of less than 1TB. In fact I have had 1 single 1TB drive fail. However I only have 20 to 30 drives over 1TB and 100 to 200 under 1TB.
Most people use separate drive for data mainly for convenience if you ever have to reinstall the OS. This means that your boot drive only has to contain your OS, and usually apps/games/etc, which rarely require more than 300-400gb, and you can save a little money by getting that smaller drive.