It’s all in your personal preferences and equipment.
Infuse works perfectly well alone without Plex and you don’t have to manage two different machines and applications.
Both can have issues depending on your equipment and combination of but they are usually minor and are corrected easily.
I tried Plex for a very short time but found Infuse to be everything I needed without having to maintain a separate application on a second machine.
I have had zero problems with 4K files over SMB.
Probably the biggest variable is the connection between the device that Infuse is on and the server. Ethernet is more likely to work the majority of the time vs wireless but both can function well if you take the time to make sure everything is set up correctly. This is applicable to both Infuse only as well as having Plex so that’s probably the biggest concern.
If you want some of the features that Plex offers that Infuse doesn’t then go with Plex. If you are content with the features of Infuse alone then why add an additional layer of possible problems?