I would not say the northeast US around Boston is that much better in escaping the heat, only marginally a little bit at best. We’ve recorded a heat index of 41C (106F) earlier this summer. Summer dewpoints typically around 17-23C (63 - 74F), so the heat index is always several degrees higher than the air temperature. Because of the humidity, you may still find it quite uncomfortable without the air conditioning, and it is rare for nighttime lows to drop below 20C (68F) in summer. I typically see temps only fall to around 72-76F (22-24C) at 10pm and it is still humid. Daytime highs typically around 29 - 32C (84 - 90F). Subtract 1 or 2C for areas far away from the cities heading towards the mountains in the north.
There is always a threat of hurricanes on the US side of the border every year in the summer and fall.
In the winter months, snow and subfreezing temperatures are irregular in areas around the major cities, though nighttime lows do go below freezing even within the major cities. The major cities tend to get rain when storm systems roll through, followed by cold and dry, but if you are lucky, there may be snow in the cities (mainly about Boston). The sun sets before 4:00 p.m. here and it is dark at 4:30 p.m.
I’d recommend Toronto or Ottawa if you actually want to escape the heat. To my understanding, they have the lowest dew points and air temperatures of any major city east of Winnipeg. They also have later sunsets and snow is more regular in Ottawa.
Get to the furthest large major city away from the equator or I’d die in 40C heat. Anything in the 40s latitude is already toast.