The issue is that the defrost mode will defrost all the ice from the fins, but it freezes again before it can all drain through the small drain hole on the bottom of the housing. It's somewhat a known issue with air source heat pumps, sadly. I've not run into this issue myself, but on a Dutch speaking forum (tweakers) there is a long thread about this modification to "survive" the cold spells with this small modification.
You can manually attack the ice with a heating gun or hair dryer to remove it, but that's a faffy.
I think manufacturers should, and probably will, add something like this themselves in the future.
Anyone on forums advocating for drilling bigger or more drainage holes?
Though, if it's snow blowing directly inside then I think creating some barrier or add additional shielding of the outdoor unit is required,so that you minimize the chance of the snow DDoS-ing the unit (note: check your unit's service manual for the minimum free distances from all sides of the unit, especially the front one that is the most important to be kept enough free space).
What if you live on the Xth floor? Your gas burner is small enough to be all inside and easily serviceable when it breaks in the middle of winter (why would it even consider breaking in summer?). Where do you put the much larger heat pump and how do you reach it?
Also, I've been wondering how much efficiency would I get if I could put some mirror and direct the light to the unit in the winter. It could help with defrosting as well.
*I was considering horizontal heat pump, but it's not worth it. It affects vegetation and future construction.
Of course it all depends on options government offers and in what place you live - however Poland I suspect also gets snowy. Just sell excess solar electricity and re-buy in winter. Just trying to save you some money on batteries.
However, at the small cottage I have for the first time just had the external units of two air-source pumps freeze solid due to snow blowing in in the current -16c spell. The defrost ran down inside the casing and formed a big block of ice in the bottom that has now reached up to the fan blades so the pump can't turn at all.
So I'm all out of heat pumps in the cottage just when I most need them :(
This is not enough to make me give up on heat pumps, but it makes me wary to rely solely on them.
Am planning a big new build, and will of course go air source to underfloor heating, but am seriously considering a backup combo wood burner for the panic week that comes each year.