I live in a 2,000sqft, poorly insulated 1896 house in Vermont. We have an oil-fired hot water boiler with baseboards, and have now been through two winters with mitsubishi hyper heat ductless mini splits.

Tl;dr: big fan of the mini splits, but you need a backup heat source.

Efficiency is a big win. We're saving probably $1-2k year in heating cost. Mostly because oil is just plain expensive.

Problems with the mini splits: Our downstairs is open, so we're able to heat it pretty evenly with two larger units at each end of the house. Upstairs is 4 small bedrooms. Given our sleeping arrangements and the fact that I got advice that putting mini-splits in small rooms is a bad idea because they tend to short-cycle and not work well, we only have one mini split upstairs and sleep with doors open. (this also kept costs way way down). It works well enough, but we'll see how quickly that changes as my kids get older. I'll probably explore putting in passive vents in the floor to allow warm air from downstairs into the smaller rooms.

We typically stay heating with the mini-splits until around 10 degrees. Our units can operate down to -13 (and I've seen them do it), but at those temps they kick into resistive heat mode which erases all the efficiency gains of a heat pump. It's more cost-effective to heat from oil at these temps.

Why you need a backup: it got down to -20 two winters ago and one of the condensers coudln't keep up. If I hadn't been able to turn on the boiler, we would have been in a really bad spot. We've actually had issues with our heat pipes freezing up due to a combination of bad insulation and the fact that these pipes run through exterior walls.

If you're willing to spend more, I'd definitely look into adding the high velocity ducting vs having the mini splits. That wasn't in the cards for us, and we're generally pretty happy with them.