Mana control / influence fun vs viability

Currently have proteus, Hansel, onatel, arial and leonidas. I switch out Leo for bt as the team is kinda soft at their current level.
Question: is the the team viable when

Ummm, short answer: Yes. All heroes are ‘viable’ when maxed.

The better question is: How effective

But even this question does not have an easy answer. The effectiveness of your team depends on the opposing team’s make-up.

I have always been a fan of mana reduction, mana stealing, mana prevention…All heroes require mana for their specials. If you can slow the mana production down at all you will have an immediate ‘edge’ over the opponent. It’s one of my favorite themes in the game. It can be problematic when it comes to their survivability as they tend to have less defense or health (always exceptions to the rule tho), so ensuring they have someone watching over their health is an important component to the mana stealing/preventing team builds.