Thank you for the answers. Then while it's not an impressive one but kind of an interesting pick.
The initial issue I have thought were accuracy and durability, for it's the artillery with only allowed to use standard HE and smoke which is infamous for its inaccuracy, and the tube's sheer weight prevents the armor to plate it with enough point. When I try to make the custom one it wasn't possible to plate it the full point of armor on them depend on some armors(at least it wasn't possible for mimetic, when I want to put ten turns worth of ammunitions with it). But are those fine? Anyway it was no wonder that both grenadier II and Centaur have such a pathetic armor points.
At least for accuracy, it could be also used for the direct fire mode, though. 17 hexes is pathetic range for an artillery, but as the direct fire weapons it's a rare sight for a battle armor, at least, which only LRMs can compete. +4 to hit is annoying at the first glance but it doesn't consider TMM either, so it's same or even more accurate than the other weapon shots on the medium range.
An another issue would be transport. Only heavy or assault can take this, and for mediums it can carry it but cannot have any ammunitions(it carries the ammunition as if it's missiles and missiles of BA must be on the same part with the tube) even if you suffer considerable sacrifice to actually managed to put it on a medium BA. It's the matter of the criticals so squad support weapon is not an option either. Perhaps magnetic clamp is a thing for this, as Centaur does?