It is a more powerful flamer. Emphasis on flamer, meaning it basically has all the issues flamers have: short range, low damage, comparatively heavy weight for what it does.
On battle armor, it is a dubious weapon because it weights so much. Not useless, if you gotta use it, and its inefficiency does make it relatively cheap. But the suits that can carry it tend to have better and/or more useful options. Consider the Black Wolf, whose plasma config offers anti-BA capability and heating option, or ER small pulse that deals more damage with better range and to-hit bonus along with anti-infantry capability, or BA LB-X for anti-vehicle work. The heavy flamer doesn't seem terribly interesting or great compared to those.
On 'Mechs, it suffers from similar issues. It is heavy for its range and damage, requires ammo. Standard flamer or ER flamer offers same utility more effectively usually. Its sole boon are the special ammo options, but few 'Mechs mount the weapon, let alone enough ammo to make use of alternate options.
On vehicles, the heavy flamer is more interesting option, because it is like a vehicle flamer and doesn't require heat sinks (IIRC). But i can't actually name any vehicles that use it...
are we talking the Heavy Flamer or the Heavy Support Flamer?
many BA weapons have different names than their mech counterparts.
Heavy support flamer is not a BA weapon but a conventional infantry weapon. The BA heavy flamer is called just that, "heavy flamer".
