Time under tension

I watched a video I can’t find again on YouTube. The video suggested instead of focusing on reps work for time. Example instead of doing 3 sets of 12-15 do 3 sets for 1 min. If I do 12-15 reps in that time great. If I only do 8-12 that is good too. It’s about slow controlled movements for the allotted time. It’s not cardio so the goal is not to knock out as many reps as possible. The idea is time under tension and slow controlled movements. 
I won’t lie. I have been trying it. When possible I have been changing the reps on Jim Stoppani’s six weeks to sick arms. My times go from 30-90 seconds depending on the rep range. My thinking was it is a great change up and would be good for muscle confusion. 
What are your thoughts? Is this a good idea? What are the benifits vs the risks? Is it a huge mistake? I have tried finding other videos that talk about this but can’t find any. 

Yes play around with different rep speed etc. slow and controlled isn’t always the best.you can incorporate other factors such as bands which should be performed as fast as possible to help recruit type 2 fast twitch muscle fibers which are the biggest and the strongest. Variety is key and there is not one specific way to always do something.