I've used a refurbished (by the manufacturer, or at least whomever they hired to do it) JBL GTO amplifier in my garage bench setup and it's never been an issue.
That said, what I think you might want to be careful of with refurbished amplifiers is crosstalk: when you can hear, faintly, the right channel's "data" in the left channel speaker when it should otherwise be silent (e.g., no audio input on the left channel). In real life I don't think you'd notice this if the amplifier had this issue (unless it was really bad, then your sound stage would narrow for seemingly no other reason). The issue is more likely due to design than that particular amp being a dud, I think (but I'm not an electrical engineer amplifier designer man). My experience in the industry is limited compared to many on this forum, so with a grain of salt, I noticed this occurring sometimes (but not even remotely the majority of times) on refurbished amplifiers I've installed.
The majority of the time the refurbished amplifier was perfectly fine and, I would imagine, reliable. I guess you increase your risk of getting a DOA or subpar performer, but I'm in the camp of not fearing refurbs (if I can return it without a f'ing restocking fee or be forced to deal with the warranty instead of just being able to return).