you do have more survivability with a healer companion, no doubt about that. i leveled my shadow tank after i completed my guardian tank's story, so i was already familiar with the role. Nadia wasn't too squishy to me, but that was probably because i gave her my hand-me-downs. she was a lot easier/cheaper to gear up than Tharan and Qy were. also if you spam slow time, mobs shouldn't even be touching her. i also chose not to level with a healer so that i could learn how to tank properly and get used to my defensive skills. i know how much abuse my toon can take before things start getting hairy, and it got me used to popping my defensive cds. whenever i had a healer companion out, i'd get lazy and not pay as much attention to things like that.
When I leveled my shadow, slow time still broke cc and as it was the first character I made on early access I had no idea she would be a willpower main stat comp so I hadn't been saving anything for her. The reason I made a shadow is in large part due to nostalgia for my shadowknight from EQ1. I've been tanking since 1999 so there was no need to get used to defensives. In fact, this game is decidedly less busy than that one with regard to constantly having to press "oh s&*%" buttons using a healer comp or not.