In this disadvantage I want to talk about VR, but let's start by talking about the screen first. One of the disadvantage of the OnePlus 5 in my opinion is that it comes with only a Full HD (1080x1920 pixels) display. It's a good display don't get me wrong, but I think that for a 5.5-inch high-end phone it would be better if it came with a 2K display. The resolution becomes more important the larger the display is. The reason is because of the pixel density. To get the crispier view, you want a higher pixel density.
Many people probably won't notice a substantial difference between a Quad HD (2K) display and a FHD one. Some even say that it's better to have a lower-resolution display for better battery life and better graphics performance. That all probably true, but I personally don't want the screen to limit me for other applications that I might be using the phone with. One of the most notable one is VR. To enjoy VR you really want the highest pixel density you can get.
The thing with VR is that when you wear the VR headset, it has some sort of magnification lenses, that makes everything on the screen appear larger. Furthermore, the screen is divided into two separate areas, so straight from the get go the resolution is split into two. This means that each eye gets to see half of the resolution regardless of which VR app or game you intend to use. Now as I said, each side is magnified, so with a low pixel density display it's very easy to notice pixelation, which degrade the VR experience is a whole.
According to GSMArena, the OnePlus5 display has a pixel density of around 401 ppi. This is not good for VR. I would say that we need at least 500, but 800 ppi or more will makes things just look much better. The Samsung Galaxy S8 comes with a 5.8-inch display, but it has a 1440x2960 pixel screen resolution which translated to approximately 570 ppi pixel density. Of course the S8 was designed with VR in mind and it needs to support Samsung's Gear VR headset to deliver high-quality mobile VR experiences.
The OnePlus 5 is also not Daydream VR compatible (source). The thing is that a specific device, in this case the OP5, has to go through a certification process to be Daydream-ready. Then you know that you can really enjoy a great mobile VR experience as Google intended you to experience and as the developer aimed its games and apps for. Even if you do able to run it, going with specs under the minimum requirement just means that you get a poor VR experience, whether it's frame rate issues, pixelation, etc. For VR it's very important, so this is not something that you just want to find ways to get around.