When I first started learning about Islam, the concept of Shi'a Islam vs. Sunni Islam was just overwhelming. I was coming to Islam from a completely different religion--Christianity--and already had a lot of different things to learn, but trying to decipher the Sunni-Shi'a split was beyond me.
I knew I wasn't Shi'a, but I didn't really know what Sunni was so I was "just a Muslim." And after spending this Eid with some recently converted Muslims, the idea of being "just a Muslim" was brought again to my mind.
For some time I have considered myself to be "Sunni," but only upon discovering what the term actually meant. Personally, my exposure to Islam (before converting) was pretty much exclusively to Sunni Islam, which is for all intents and purposes "Orthodox" Islam. That's not everyone's experience--some people come to Islam through Shi'a Muslims, and might get very confused by the different experiences between Sunni and Shi'a. Something such as the prayer, which can be confusing even if a person is only seeing one viewpoint, can be drastically complicated with exposure to Shi'a customs as well.
I don't think there's any problem with Muslims converted to Islam and saying "I'm just a Muslim" to avoid Shi'a/Sunni confusion. But I do hope that they learn that Sunni Muslims are "just Muslim" too. It might even be simpler to just say that they are only "Sunni" to clarify that they are not Shi'a, without forgetting that "Sunni" Muslims account for the immensely vast majority of Muslims worldwide.
It may be that avoiding confusion entirely isn't wholly feasible, but simplification should undoubtedly improve understanding. I'm a "Sunni" Muslim because I'm "just a Muslim" too, a follower of the Prophet Muhammad ﷺ, and one part of the body of Muslims who also do so.