To be honest, I was disappointed to see that the OnePlus 5 doesn't come with a microSD card slot. The base model comes with 64GB of internal storage and the higher end model with 128GB. The good news is that there is that the storage capacities are very generous. The bad side is that many uses want the option to have an option to use a memory card because this is a cheaper way to enjoy high capacity storage. If you want the 128GB, you'll have to pay around $60 more for it, whether a 64GB microSD card will cost you much less.
I personally enjoy shooting 4K videos and the OnePlus 5 does support 2160p30 4K UHD video recording. Just so you know, or you probably already know, that recording 4K videos takes a lot of space. In the past I had phones with no storage expansion option and I always needed to keep an eye on how much storage is left on the phone when I was outdoors recording those high-res videos.
Of course if money isn't an issue, the 128GB will probably be sufficient for any hardcore user and many people will also be satisfied with the 64GB version as well. It's not a major disadvantage, but still, I think that having an expansion slot would have been very useful for users who are going to take advantage of all the multimedia capabilities of the phone and prefer the option of buying a cheap microSD card (Which also gets cheaper every month) instead of investing $60 more to enjoy double the capacity.