Battle High/Fever system is fine. It's meant to reward you for 'PvP'ing' smartly and having the sense to be escorted/being near healers (or having healers at all). If your team possesses multiple Battle Highs and even Fevers, that is an indication of your team's overall performance throughout a game. The increasing momentum and ease of killing targets is your reward for not having a team full of over-extending people that aren't moving with the group and end up being picked-off as easy targets. It's not a crutch, it's an indication that some/most of the people on your team are doing something right.

I don't think both bonuses are meant to be a disadvantage. They are an incentive for the other teams to get rid of players with this bonus, but at that point those players likely have 25% extra or more of their healers' attention.

The fact that Frontlines is a 3-way team conflict is the comeback mechanic. You always have the means of shutting-down any one team (in the lead) if both tactically sandwich them. The problem is this comeback mechanic is largely designed like the Prisoner's Dilemma. Do you work with the other team to take down the team in first place/ with a lot of Battle Highs & Fevers? Do you sit back and watch one completely trash the other and get Battle Highs & Fevers on the basis of controlling objectives (which might not even be enough)? Will the other team instantly turn on your team instead of continuing the double-team? Many times I have seen people wrongfully call-out "Let them fight" in situations where it was in our best interest to pincer the dominating team- only to be overwhelmed later when they finally turn their crosshairs on us.

If any one team starts getting a ridiculous amount of Battle Highs & Fevers, chances are the fault lies with the two losing teams due to at least one sitting-by and allowing it to escalate or by not creating a temporary truce to put pressure on them and 2 v 1 them.