It's heterodox because they believe all sorts of things under the same label. This concept is not alien even to the post-Vatican II Catholic Church. Heterodoxy is manifested among such (nominal) Catholics as Nancy Pelosi, or some of the Wiccan nuns in pantsuits. They sort of make their own doctrine on various dogmatic issues and still claim to be Catholic. This is, of course, incompatible with Catholicism but not with Protestant sects/cults.
Thus, although they are relatively few in number, the diversity of beliefs doesn't lack among Messianic communities. Not all so-called Messianic "Jews" believe that Jesus is God. Not all of them observe the same rituals or dietary restrictions, etc. Like most Protestant communities, the doctrine is "winged" to an individual taste.
But this heterodoxy is not limited to fringe groups only. It can be found in mainline Protestant denominations, I would say in abundance, although most Protestants will tell you they haven never really encountered anyone with far-flung beliefs among various Protestant denominations—certainly not "doctrinally significant." (Mr Rogers comes to mind).
For instance, here you have an Episcopalian site which openly doubts the divinity of Jesus. I find that "doctrinally significant."
“although most Protestants will tell you they haven never really encountered anyone with far-flung beliefs among various Protestant denominations—certainly not “doctrinally significant.” (Mr Rogers comes to mind).”
Horse pucky! I’ve always specified Sola Scriptura type churches, not Protestant. I am well aware that many mainline Protestant churches couldn’t find a Bible using a flashlight & a map, let alone read it or believe it.
And among churches that actually believe Sola Scriptura, the doctrinal disputes are primarily charismatic vs non, infant baptism or not, and predestination vs free will. And my experience with charismatics, including in my family, is that they don’t go with sola scriptura, since their experience is their real guide. And lets face it - it is really hard to argue that scripture AFFIRMS infant baptism!
So that leaves PreD vs FW...