I am going to let you in on a little secret. Opinions are like genitalia. Everyone has them and some people are always whipping them out in public and making exaggerated claims based on opinion and not on fact. This applies to both the people who say only positive things as well as those who say only negative.
I have used multi-pedals and modeling amps from the following companies:
Line 6 - Spider Valve Mark I, Spider IV, POD HD 300, POD HD500, Amplifi, and FireHawk. (Currently Looking at Spider V) VOX - VOX VT+ BOSS - ME-50 Zoom - G3, MS-50G, and MS-100BT (Currently waiting on the B3n) Peavey - VIP Fender - Mustang v2
I can tell you a couple things from my experiences. Firstly, a modeled amp will not sound exactly the same as the amp it was modeled on no matter what anyone will tell you. Many of the modeling products will come close enough to the original that the vast majority of people couldn't tell the difference. Secondly, and this is the most important thing. If you take the time to learn a product you will be able to get a good tone out of pretty much any modeling amp or multi-pedal.
Ok, now that I got that out of the way, let's address the amp. I have played around with the Spider V and I really like it. It has a huge library of amps and effects built in and plenty of other useful features that will make learning the guitar easier and more enjoyable. As a beginner, this amp will benefit you greatly. It will give you an opportunity to learn about a wide variety of amp types and pedal types. It is easy to move pedals around to see how that changes the sound. You will learn about how the different types of amps sound, (not brands but types, Clean, American, High Gain and British). It is the easiest way to learn the difference between a Chorus and Delay without having to spend extra money.
As if this wasn't enough, if for some reason, you end up hating the sound of this amp down the road, you can use it as a powered speaker for other multi-effect pedals or a traditional pedal chain. Many manufacturers are making quality analog pedals based on different types of amps and you can use one of these as the amp tone adding whatever physical pedals you desire...or can afford.
As you have pointed out, Line 6 has been in business for 20 years. That in itself isn't proof they produce the best products but it is a very good indication that they make products people like. As a beginning guitar player, it is far more important you like the product than what some reviewer on YouTube says. I am quite confident you will like the Spider V. YouTube reviews should be taken with a grain of salt. It is far too easy for a reviewer to be biased against a company and not put any effort into the product to get the best out of a product. The flip side of this is you can't be sure a reviewer is not being paid for a good review either. The only review you should trust is yours. If at all possible try the amp out yourself. Download the Spider V app and play around. Watch some videos on YouTube but make sure that you watch as many positive and negatives as you can find. I am going to catch heck here but I would compare the reviews to the Line 6 videos about the Spider V. Why would you do this you ask. Well, YouTube is full of crappy sounding videos and it would be easy to distort your view of the amp. The Line 6 videos will show you the best the amp is capable of.
Oh...one last thing, too many people talk about the built-in patches. It has been my experience that the vast majority of the patches a product builds into a product are to help show the different sounds the product is capable of.