I95 in MA and CT (and presumably RI) can have a fair bit of traffic; in my experience it usually moves along okay, but there's a lot of vehicles on the road. Not my favorite driving to be sure. I'd probably take I-90 (toll) to I-88 to I-81 rather than I-287 and I-78. I-84 through Connecticut is not bad now (there is still some construction going on, but the worst of it is past and the road in decent shape); through NY and PA it's pretty rough pavement, at least as of a year or so ago when I was last on it.
Depending where in Maine you are, it might also be sensible to take e.g. US2 across to Montpelier, Vermont, then I-89 to I-189 to US7 south to VT 22A to US4, into New York state, and 9N to 149 to I-87 south and continue on however seems appropriate. Not the fastest route, and not perfectly flat (you're going east to west in northern New England, after all), but nice roads and not bad traffic.