That video shows how far from regular D&D the game is currenty. The character always starts by being invisible and 'surprising' a creature (often with a poisoned weapon, with the best poison - of course). And with the non-existant backstab (for non-rogues), which does not exist in 5E. Invisibility is a powerful ability or an expensive scroll/potion which should be uncommon - because it is unbalancing.

In actual D&D getting surprise every round is pretty much unheard off - it smacks of poor encounter design or an overpowered character, which the DM would manage. If you know players are going to be invisible all th etime, then you take steps to counter that, to even the odds....

Whatever you do, something/one else can do too...why couldn't a monster or NPC (esp in the underdark) be lurking about invisible and see your character walking/sneaking about and then do the same to them? Clearly beyond the AI/encounter design at the moment and exacerbated by making things bonus/free actions that should be a full action.

Also, you can still sense invisible creatures (and many monsters can or should be able to), that never seems to be an issue for our invisible hero. The bullete has tremosense, which means you are not actually invisible to it when 'invisible'. It doesn't need eyes to sense you - that is part of what makes it a more dangerous foe. So simply walking up to it invisible - without it noticing you, unless you are sneaking - should not be possible. Fighters with armour (almost always) have disavatange on sneak. In fact, I would as a DM give the bullete advantage on it's perception check in this case - being in it's own env. But even without that, it should 'notice' the character.

Anyway, after the opening guranteed 'surprise' round, guaranteed first attack in next round - so they always win initiative apparently - again, not that common, some creatures just have very high initiative scores and with RNG you must lose some times...anyway, not here it seems. Then abuse of the broken 'backstab' again as a non-rogue to get advantage on their next attack roll, thus near guaranteeing a hit. Then action surge, which is legit (but which requires a long rest to recharge - so a resource cost, which is completely ignored since you can long rest whenever) to effectively get a third attack (with advantage too, again not an actual rule). So 3 attacks, with advantage, from a low-level character against practically any foe, before they ever get a chance to strike. It's no wonder they all die before they get a chance to attack. And if you attack the boss first, and abuse the use of spell scrolls/bombs etc mopping up minions becomes trivial I imagine.

Sure, some of this is legit, but generally the rules of 5E are balanced to avoid such one-sided encounters; here every encounter is massively tipped in favour of the character.

I didn't watch more than 5 minutes, because appears to be the same strategy applied repeatedly. If the point is to show how the rules are currently exploitable, this works magnificently. All this shows is (1) the 5E rules as implemented now are broken/exploitable (if you take the time to find those holes of course) - this is known and has been pointed out repeatedly on these forums (2) that there are too many resources (esp scrolls usable by other classes) around for players to exploit. Hopefylly Larian is paying attention and will nerf the exploits instead of just giveing everthing more hp/AC when some people complain about it 'being too easy'.