It's a great unit in quite a good commander too, will be less effective after patch when you can't use UC as command vehicle, a change I strongly disagree with but that's another topic.
Unlike ST it can't be decrewed and reloads a lot faster but has a shorter range, a smaller AOE and cant shoot through shot blockers.
It's cheap for what it is too (Use this for reference:
https://www.coh2.org/guides/40402/british-forces-unit-guide) at 140 fuel BUT a high 600 MP although with the squad wipes you should get from it, it will pay for itself effectively too.
It has High damage vs tanks, you just have the hard task of... hitting them as the shots can take a while to line up and fire (a change to the AVRE in WBP will make this slightly faster at the cost of more micro with hold fire added).
Basically the commander is most usefull if you have emplacements, one ability allows you to repair them remotely so they can still fire and repair, even repair when braced and can make them very hard to kill. Combine a mortar pit with the next ability, the Command Vehicle and its recon and you have a very powerful tool.
The anti garrison flame mortar is a useless ability, more so when you should have a mortar pit anyway and its so situational that it mostly goes unused, just seriously forget about it, unless of course, building MG spam.
The reason it's not used more is because its overshadowed by better commanders, LM does just a good if not better job at wiping squads and doesn't need to be into danger to be useful, you get infiltration commandos (your recon equivalent) and flamers on your engies (your anit garrison flame mortar equivalent but 1000x better).
Or you have the Vanguard with the Croc that kills constantly, can engage both tanks and infantry at the same time, Air support and an OP Officers charge. If they added a different ability than the flame mortars for something that could actually be used when there are no buildings I imagine it would be used a lot more as that would cover an indirect hole that the commander lacks, but then again, it might make it too strong.
TLDR; Overshadowed by better commanders but in no way a unit/commander to turn your nose at.