You'll want to be sure the driver has the proper documentation to cross the border.
Yes, you may run into problems. It may also be illegal. I would worry about issues at the border. Not only for you but also the driver. Not all drivers hold valid passports. You may also seem suspicious.
It would be better taking a train. Are all the buses really sold out? Any way to make a connection?
And expect to pay much more than 100$. No way. Too cheap even for uber. Also expect wait times at the border crossing. Between uber and taxi I thin ktaxi is the better option because some of the drivers are used to it.
There are some taxis that will do intercity travel. You can request one at the airport. Wait in line at the taxi wait line. Tell the airport staff your destination. They can radio a taxi from the wait area that will do intercity travel. I know some go as far as Ottawa or Quebec city. I don't know if all will cross the border. You may have to wait for one. If you are not leaving from the airport you can call one. Or have your hotel call one for you. Be sure to specify where you are going.
Personally I would not risk getting denied entry. Or being put on a watch list for minimum 7 years. All because of an uber. That is if any uber will take you there. And we haven't discussed the problems with uber insurance yet.Edited: 26 July 2018, 02:44
How would I end up on a watch list with a completely valid passport? Also what driver would attempt crossing a border without a passport? I mean this requires about 10 seconds of due diligence. I'll be in Montreal for a few days and I'm sure I will be in a dozen Ubers in that time, so I guess I'll just ask drivers what they think.
I would be very wary of using Uber, their quotes are notoriously off.
Plus you need a driver who can enter the US. Lots of Montreal drivers are foreign born and may need visas for entry into the US.
Look at the taxi firm linked above.
It's not just a matter of the driver having a passport...technically the driver would be working in Canada without a permit, if caught at border, entrance could be denied. Why don't you save yourself some trouble and use a proper transportation company (taxi, train, rental) instead of crossing the border with a total stranger and risk trouble.Edited: 27 July 2018, 04:01