I've never seen anyone fall and be knocked unconscious when a streetcar did a sudden lane change.
I'm not sure the REM is really that comparable to Ontario Line. It's just that we'll integrated into the urban structure, other than Central, McGill, and the Blue Line interchanges.
The Ontario Line is much more urban, more comparable to existing Toronto and Montreal subway lines. At least the first phase. There's certainly a lot more planned, potentially a loop around Toronto up to the 407 and then west to Pearson and back downtown. With branches to Oshawa and Oakville. That may be bigger than REM, though I don't know what future phases are envisioned there. Equipment might be similar, but not the network and operations.
Much of REM is more comparable to the 5 (maybe 6 if Milton gets funded) GO rail lines they are upgrading to electrified frequent service with more urban stations. Lakeshore GO will be a very different beast with trains every 4 or 5 minutes.