Is such a fast pendrive a good choice for a system disk?
Fast is always good. Based on the sequential speeds alone, there's no reason to not choose it.
Overall however it's hard to tell, because as you've said yourself, random and small-block I/O performance is quite important, and you don't have that information available.
It is however usually possible to find the appropriate information online if you look hard enough - particularly common applications such as IOMeter, AS SSD, and CrystalDiskMark are used for these purposes, and searching for, e.g. "Crystaldiskmark (name-of-drive)" may give you information on the random R/W performance.
As an example, this gives information on a Lexar P20 drive, this provides similar info on a Sandisk Extreme drive, and this provides information on a fairly mediocre internal SSD, and this provides results for a fairly dated, low-end internal SSD. In comparison, 7200RPM spinning hard drives can be found here
In general, what you're looking for is 4K random read/write performance:
(Source)
In the above picture, you can see two USB drives both with very fast sequential R/W performance but one provides 0.05MB/sec random write while the other does 10.2MB/sec random write. If you refer to the five Google searches above, you'll see an old, low-end internal SSD still does around 10MB/sec random write, whereas fast modern drives do >40MB/sec read and write (internal ones often write faster than they read in random I/O). Spinning hard drives on the other hand have random 4K speeds of 0.5-1.5MB/sec in comparison.
In general you'd want to look at a USB drive that has random read/write performance of at least 3-5MB/sec. Anything above 10MB/sec is very fast as far as USB drives are concerned, the 0.05MB/sec of the Lexar in the example above is pitiful. Since running Windows is more read heavy than write heavy, 4K read is more important than 4K write. So again referring to the picture above, even the pitifully slow USB drive has 7MB/sec random read, which is 5-10x faster than a traditional hard drive.
Bear in mind, Windows To Go is specifically optimized to run from USB flash drives, so it won't be quite as slow as if said flash drive were used with a "normal" HDD/SDD standard install.
