I’m a keen hobbyist growing pears in a very harsh central Alberta climate. How harsh? -42C has been my coldest. My average winter low is -37C. This winter for example it hit -38C in late December as the coldest. So the rules for growing pears for me are 1/ They must be able to survive -40C. 2/ They must be able to survive -40C 3/They must be able to survive -40C. There are a few more main rules, but I bet you can guess what they are 
I have experimented with several pear rootstocks, and the following are my comments:
Siberian Pear, Probably the best all around rootstock for my area. It does not show any incompatiblity issues with the pears I grow, basically of Russian origin. These all have Ussurienses (Siberian Pear) in their ancestry. This is a very hardy rootstock, and makes a full sized tree ultimately.
OHxF87, OHxF333, OHxF97 These survive fine due to the aways decent snow cover we get up here. The problem is these rootstocks are always imported from the USA, and to get into Canada the roots are heavily fumigated. This literally means 4 out of every 5 of these rootstocks die on me the first year. I suppose if there was a Canadian supplier this would not be a problem. But the 20% that survive the fumigation seem to work well.
Saskatoon This is the ONLY rootstock I have found so far that actually gives a bit of extra hardiness to the grafted pear scion. I use Smoky Saskatoon, which grows to about 12-14 ft. tall. The grafts MUST be done at chest height, as strange as that might seem. Grafts near the ground take, but then die in a year or two. Why? Part of the great unknown…The pears are full sized and of normal taste. You graft onto trunks of the Saskatoon that are about as thick as your thumb, usually a bark graft, and at chest height. The only problem is the way Saskatoons grow. The trunks only last a few years, then they die, to be overtaken by new shoots coming up from the roots. My oldest pear grafts on Saskatoon are about 8 years old so far, and they may last 10 or so years. Naturally when the trunk dies, there go your pear grafts.
Cotoneaster (lucidis) This is the very common hedge plant grown in zones 2 and 3. Tests in Russia show long term compatibility with pears, 60 years and still going strong. However you MUST graft a foot or so above the ground and allow 2-5 branches of cotoneaster to grow directly under your pear graft. Why? Because the pear is great at sucking up nutrition from the cotoneaster roots, but pretty poor at sending anything back down. If you don’t allow “feeder cotoneaster branches” to grow, the roots don’t get fed properly, and in a year or two the roots die. You get an 8 ft. tall pear tree ultimately using cotoneaster as a rootstock. Fruit is normal in every way in terms of size and taste.
Aronia Berry The benefits are extremely early fruiting, sometimes one year after the pear graft is made! The downside is this is not a long lasting union, perhaps 3 or 4 years max. However Aronia is used as a rootstock in Russia to “speed up fruiting of new pear varieties”. I only grafted a few varieties of pears to aronia last year. All the grafts took and look good! Too early to tell if I will get fruit the first year after grafting. The only purpose I can see to grafting pear to aronia berry is you get to taste the pears in 1 or 2 years from grafting, vrs years longer grafted to other hosts. And you will have to be philosophical when the graft fails in 3 or 4 years due to longer term incompatibility.
Hawthorn and Mountain Ash. All my pear grafts experimentally to either hawthorn or mountain ash take really well and grow great, the first summer that is… Then for some reason they simply do NOT leaf out the next spring. Perhaps it is my cold winters, or perhaps the varieties I’m using are not compatible for the longer term.
Pears growing on apple trees via interstems. I have done a fair amount of experimenting with growing pears on my hardy zone 3 apple trees via a Winter Banana or Palmetta (an apple crab bred in Novosibirk, Siberia, extremely hardy) interstem. Again they all take very well and grow normally, actually better than normally, the first summer. However half don’t leaf out the next spring, and the other half only last two years. The longest lasting has been 3 years. My guess is my severe winter temperatures somehow damage the graft union, and this is why the interstem fails on growing pears on apple trees. I understand in more mild climates pears can be grown on apple trees via a Winter Banana interstem, but this doesn’t work in my climate. While I still graft pears to my Palmetta apple crab and they do very well the first summer, I only use this to grow scions and I harvest them in November and store in a buried 5 gallon bucket over the winter. If I harvest the scions in late March, often they are dead.