First and foremost, tank rell is dying thanks to the troglodytes at riot nerfing her constantly due to her performance in proplay. Her CDs are just too high to make her a valuable enage tank when you don't have the perfect comp (I.E Miss Fortune ADC).

When you hit full build around 30-ish minutes, your E deals at LEAST 30% max hp magic damage on hit, plus 700+ from Q, plus 600+ from W, plus 1k+ from ult. Your full combo deals over 3k damage in the span of 2 seconds on average.

I've been running AP rell for around 2 years since her rework (may full ap jungle rell rest in peace) and I cannot FATHOM going back to tank, AP rell is just too much fun and it's wholly viable as rell is not the best support for how tanky she is, but how much utility she has for her team. As stated before, Rell is somewhat reliant on her team being able to synergize with her via big aoe ults and other abilities (hence why Miss Fortune is the Holy Grail of ADC picks for you as support).

Overall, the build is just more fun than tank rell, you feel more like a member of the team and not just some tool to be used for others' glory.

AP rell is stupid fun to play if you don't mind the effort to get big damage numbers, but if you fall behind or if your team is losing, they will blame you since you didn't run tank. As an experienced Rell player, all I can say is you do you, this build has it's strengths, weaknesses, and counters just like any other build on any other champion--no one is safe from getting counterpicked. Keep your head up and focus on the next match if your current one is essentially doomed, and really, if you get Lucian...just dodge and take the time penalty, save yourself the headache.

Enjoy this build while its here, there's no gurantee we'll have it forever, especially if someone in proplay starts running it and wiping teams like I do on a regular basis. Au revoir.