Welcome Samuel
I don't know if you read many Internet reviews but I have because I like to do some research before I buy. Go to a place like Amazon that has many customer reviews and pick a product. It doesn't matter pick a camera, TV, carpet cleaner, coffee maker. Even when you find one with between a 4 and 5 star rating you will find customers who rate the product one star and give you a list of all kind of problems. So what's the answer. Great product or junk.
I have no idea if the Onkyo 6300 has any major HDMI issues. I have heard that some do not like the performance of HDMI upscaleing, and that they tend to run hotter than other units. But when I walk in a retail store and a salesperson starts bad mouthing a particular brand it sets off a red flag. If it is a piece of junk why do you sell it? Or if you don't sell it are you trying to bait and switch me to your line products.
Are you taking about an Onkyo receiver or a Onkyo HTib, as the s6300 sounds like a HTib, If this is true I could see a salesperson trying to sway you away to another brand if they don't sell Onkyo HTib. I think you will find that Onkyo HTib are among the best HTib on the market as they come with a real receiver, a real sub, and speakers that can be upgraded. Perhaps the only thing that might be close is a Denon HTib. If you have your doubts about an Onkyo s6300 then you should be looking at a different brand.