can i use Samsung x5 ssd for booting imac 2017. Is it equivalent to internal ssd ? what are pros and cons

I am using a Samsung X5 this very moment as a boot drive. I reformatted to APFS with a GUID partition map before installing macOS Mojave. The X5 works very well. As far as performance numbers, I do not know. It seems pretty fast, it responds as fast as my internal SSD drive.

You've really not provided enough information to accurately answer your question. For example, you've not mentioned the size of your internal SSD. However, a few comments... "Identical to internal SSD?" Almost assuredly not, but "similar". That depends on your point of view. Apple does source components from different manufacturers so it would be hard to say how similar without knowing the exact drive that is in your Mac. A lot of users do boot from external SSDs and it works extremely well for them. And a Thunderbolt 3 drive should be very nearly as fast as an internal drive. You would need to clone your system to the external drive with "Carbon Copy Cloner" or "Super Duper!" and boot from there. You could then use the internal drive for storage.

its not even close. Just trying to remember the numbers but it will give you an idea Write 2400 m/sec to 300. Read 2859 m/sec 389

Its good enough though to boot and use as an external.

I would be concerned with the power usage and use ASP to make sure its within power requirements. Look under USB in ASP.

I'd also be concerned long term over trimming ie garbage collection. If it was a thunderbolt device then you could use trim force command to deal with it but I would not use it in Mojave.

Other than that is ok but I prefer thunderbolt devices.

I think you will be quite happy with the X5.