The problem with Christian heresies is almost exclusively a problem with intellectuals, especially professionals.
(Of course, the main problem is determining who is the heretic; since both sides claim to be orthodox.)
But, to take the example I know most about, Mormons could be regarded as a Christian heresy - what is the problem?
The problem relates to several of the second-order aspects of Christian doctrine: it is mostly a matter of theology.
Because in terms of actual behaviour, Mormons are pretty much indistinguishable from other types of Christian except that they are more devout than average Christians (i.e. more 'Christian' in their behaviour, to use common language).
But Mormons have no professional priesthood (or, more exactly, a trivially small proportion of professionals, who are nonetheless very important), so a comprehensive and consistent theology is of little importance to them; and theological limitations (or heresies) - incompleteness, contradictions... have little impact.
At least, I find it difficult to observe any particular problems which have arisen from Mormon theology over the past 180 years.
The main thing about Mormon theology is that the heretical aspects (heretical from the perspective of Roman Catholicism, especially) arise becuase Mormon theology is very concrete (not abstract), very narrative and time-bound (not focused on 'eternity'), very close-up and personal (not philosophical).
Mormon 'heresies' are therefore not so much deliberate deviations as the natural consequences of re-expressing Christianity in concrete and temporal fashion for the plain man.
Mormon theology is intrinsically realistic and narrative in style and concepts, and could not express the subtleties of Catholic theology, even if it set out to do so - which it does not.
For example, instead of the abstract, mystical and intricate conception of the Holy Trinity (e.g. as expressed in the Athanasian Creed), Mormons have God the Father and Son as separate actual persons.
From a theological perspective, this is heretical and incorrect; but the accurate Christian conception of the Trinity is - well - very difficult to understand; very abstract, very mystical.
And without a professional priesthood, and a few hundred years of theology, this kind of abstract conception cannot ever develop or survive.
My point: there are gross and deliberate heresies which must be resisted, but many heresies are more like re-expressions; and the people who are most at risk from heresies are therefore intellectuals and religious professionals.
Indeed, for intellectuals and religious professionals, there is no form of orthodox Christianity which is heresy-proof - intellectuals can make anything into a heresy, and lead others down the path.
A heresy is like a fork in the road - but some heresies fork-off then go in-parallel with frequent crossings-between; other heresies lead further and further away from the truth.
Looking back to 1830 when Mormonism was founded, we can see that it has not 'strayed' far or indeed significantly, and (except theologically) it does not look as if there are any real barriers between Mormonism and orthodox traditional Christians.
But orthodox traditional Christians are (in terms of power) no longer the mainstream.
Liberal Christianity, which began to develop at the same time, was not obviously a heresy for many years. Indeed, since it has captured most of the intellectuals and theologians and professionals in religion, Liberal Christianity sees itself as the mainstream.
Yet, Liberal Christianity has - from heresies so subtle as to be hardly perceptible for many decades, heresies embraced by the many or most of the leading theologians and intellectuals, by now diverged so far away from tradition and orthodoxy that it rejects all of Christian history up until a few decades ago; it also rejects paganism (Natural Law) and has nothing in common with any other major religion.
So there is this about heresy: that heresy which seems clear and gross from a theological perspective may be of trivial significance, indeed have some very obvious benefits - while subtle heresy may lead to a situation indistinguishable from total apostasy.
The main lesson is that theology is not Christianity; and that for most Christians throughout history and around the world, their 'theology' is necessarily very simple, concrete, common-sensical and story-like - and therefore (from a philosophical perspective) necessarily incomplete, inconsistent and inaccurate.
But how much does this matter?
Wrong theology may lead a Christian significantly far astray - but not necessarily. And perhaps it is seldom the wrong theology which does the leading astray; the problem comes when the desire to stray distorts theology, and the resulting distortion may be very subtle indeed - imperceptible, at first, from the intrinsic inconsistency of human affairs.
But a simple Christian with incorrect theology may be, often is, and historically usually has been a better Christian than the theologically-correct intellectual and professional.