This sounded a bit crazy to me, I'd have to pack all of my stuff in my apartment and hope I get a T1 when I arrive at Pearson airport in Toronto? And if not I'll have to find some place to stay.
My company's lawyers went through all the "yes this person does deserve a visa" stuff for me ahead of time. When I got to the border crossing (also at YYZ Pearson) I had paperwork that made it a much faster process. I'm comparing this to the previous TN-1 I had which required a lot of questioning, waiting around, etc. If you've got the extra time, I really recommend trying the pre-approval route.
The guys at Pearson are absolutely humourless[0], but they're just doing their job. Neither time did they play any games or use trick questions to try to deny me. (I've heard the ones at the Vancouver land crossing sometimes does).
[0] I make jokes when I'm nervous. They did not appreciate that.
My own anecdote is I tried to go through the border before officially moving and get the TN and it was a disaster. I got pulled into a long interrogation with a surly border agent. He told me right from the start that he would refuse to officially process my application but would review it for my benefit. He then spent about an hour going over it while I waited then came back with some faults he found in it. He told me I was lucky he didn't process it since I would have ended up with a hard-rejection on my official record.
I got in contact with the companies lawyer who had prepared my application and she told me that the agent was likely inexperienced and wasn't sure how to process TN correctly. I was ready to call it all off and then they offered to have a priority process pre-approval through DHS in Virginia. I don't think they still offer this but at the time you could send your TN application to the DHS head office and in 1 week they would give a determination on eligibility for TN. It cost a few thousand to do it. You get a form as a result (not an actual visa or even a guarantee of entry on TN) which you take to the border along with your application. It attests to your compliance with all necessary pre-requisites for TN entry.
When I went to the airport (YVR in my case) they have agents specially trained in TN processing. My anxiety was still extremely high since now my apartment was gone and all of my stuff in a truck. I showed him my application along with the pre-approval form and he smiled, stamped something and said "Welcome to the USA". Took less than 5 minutes.
Since then I've gone through the process 3 or 4 times, each subsequent time without the pre-approval. Each time it has been thankfully painless. My advice: go to borders that have optimized processing and make sure your flight aligns with their times of operation (see https://www.cbp.gov/travel/canadian-and-mexican-citizens/tra...). Have a very professional looking application. Each time the company I worked for prepared a multiple page application with everything extremely professional looking and I presented it in a plastic cover. As soon as the agent sees you have come prepared they always seem to go through it quickly and they usually are very happy. Make their job easy and they will make your entry easy.
Watch out for sly questioning. They often drop in questions like "Where do you live?" (correct answer: in Canada), "What is your job?" (correct answer: Exact job title on your TN application). Make sure you bring originals - I think they have training to look out for copies of e.g. diploma, proof of job history, letter of job offer. One document I had was signed in black ink and they looked very close to make sure it wasn't a printed version.
Despite how careful I was, I always had anxiety/stress going through the border while I was on an TN. Any agent at any time can just decide to revoke your access and then you are stuck. This can happen even when you travel during an already approved TN stay. The company I most recently worked for started sending warnings to people going through the border on any kind of visa since the current administration is leading to higher rates of refusal of entry. For this reason I avoided leaving the US while I was there.
IMO, luck in getting the right agent plays a role. I was often very luck (after my first disastrous encounter), getting someone who was clearly in a good mood or had positive sentiments about the company I was going to. I was also lucky with my timing, as in there weren't long lines of people before or after me. These kind of uncontrollable aspects to the process are part of what makes it so stress inducing.