We had an under hung 115 kV arrestor fail several years ago. At the time, there was vigorous debate as to whether the mounting allowed water to penetrate the joint between the sheds and the base. Unfortunately I don't remember if there were definitive conclusions.

Good point.

These SAs were porcelain or silicone polymer housed?

The insulator shed design will likely be different for under-hung. Consult the manufacturers manual for details of permissible mounting angles. Avoid using the arrester as a jumper or dropper support so that it can be disconnected easily if it fails, and the main power circuit can quickly be restored to service.
Regards
Marmite

Agreed with the above in terms of the possibilities of failure and the ease of replacement. We recently installed underhung arresters due to a limitation of space. The preference is to go with composite housings, but some utilities do not accept these. The risk of catastrophic failure is higher with porcelain housings over composite housings. The shed profile will be different,as explained above, with the shed angled towards ground in its inverted position. The arrester grounding lead will be physically (unless it's insulated?) connected to the frame of the gantry, and if I'm not mistaken, this will lower your inductance and lower your resistance compared to an insulated ground wire.

Good point qman5

I think the utility will require an insulated conductor down to ground grid but what if the gantry frame is used as a down conductor? I doubt the inductance and reistance won't be predictable compared with a physical lead.