Old somewhat returning player here! Is it worth it to buy up cards with gems in the new system?

I have maxed out my main deck + an additional 10 or so cards mostly by getting the necessary cards from 5-star boxes. I am now considering using gems for my secondary deck, which would still require upgrading a level 15 epic to 16 and a level 14 legendary to 16. Is there an agreed on consensus on the current best ways to use gems, and are some upgrades more sensible to use gems on than others?