There are several factors that go into the history of HAV vs infanttry in this that get overlooked since the last character wipe. It all boils down to one statement as to why tanks were considered OP, and are considered UP now:

Players didn't used to take AV seriously

Dust 514 rewards players for specialization. While tankers invested 100% of their SP into HAV, they got better and better while there were so few players that invested in AV. As more and more SP became available, it only got worse. Tankers could pubstomp ambush matches all by themselves, even under the flurry of militia swarm launchers with nothing to fear. The occasional person with standard AV grenades or higher tier forge guns could pose a problem, but the rest of the tanker's team could manage that if they weren't supported properly. The tides of a skirmish could be decided by a well fitted tanker with 3 squad mates.

Was it imbalanced: yes.
Should tankers have been nerfed because they were playing their role as intended: kitten no!

Infantry cried OP and nerf, pointing the blame at CCP for the imbalance. Truth be told, they had no one to blame but themselves. If an effective counter so something exists and the majority of players ignore it, it shouldn't be considered OP or get nerfed. Tankers have been doing what they were supposed to all along: Get better at their field until they can be the most effective in that role as possible. If their counter to that role doesn't specialize to keep up, then tough kitten. There shouldn't be any special treatment.

Now, players are finally taking AV seriously. Just the other day, someone decided to drop a marauder into an ambush. Instead of everyone fleeing, I saw swarms of AV grenades, forge guns, swarm launchers coordinating to ruin that unfortunate pilot's day. When it got blown to hell, I smiled as the person who showed up on the kill feed was using a prototype swarm launcher. A few seconds after, the unfortunate pilot rage quitted before his flaming wreckage even disintegrated. If he had a squad communicating with him, or there was another tanker on the team to divide the damage between each other, the outcome could have easily gone the other way. As of right now, HAV need infantry support as much as AV need to coordinate with weapons to match or surpass the HAV's tech.

Everything is working as intended.

...But wait, there's more.

Obviously the rise of HAV didn't go ignored. More and more infantry players probably figured it would be in their best interest to go with the biggest gun. Unfortunately, it takes a lot of SP skill, and player skill to use that gun efficiently. It seems like more and more tanks are getting called in and instapopped since AV is taken more seriously and these fledgeling tankers just don't have the SP/skill to use their vehicles properly, thinking that they're supposed to be invincible.

Again, working as intended.

One thing that easily gets ignored for both pilots and infantry is core passive skills. Having a better dropsuit or vehicle doesn't mean it's that much better if you don't have simple skills like circuitry, combat engineering, field mechanics, shield control, shield boost systems, weaponry/turret operation, and dropsuit command. (somewhat optional if you're not a scout) Having all these maxed essentially give everything you use:

+25% base CPU for dropsuits and vehicles
+25% base PG for dropsuits and vehicles
+25% base Armor for dropsuits and vehicles
+25% base shield for dropsuits and vehicles
+25% base passive shield regeneration for dropsuits and vehicles
+10% base damage for all handheld weapons/all turrets (for turret operation)
-25% scan profile for all dropsuits (optional depending on role)

Pop quiz:

-HAV, would you rather have a Sagaris/Surya without core passive skills maxed, or a Gunlogi/Madruger with core passives maxed?
-Infantry, would you rather have a VK.1/VK.0 fitting without core passives maxed or a Standard/Advanced fitting with core passives maxed?

I'll leave these unanswered since the obvious choices is... well... obvious.

For a more economically sound and efficient merc:

I don't specialize in HAV, but this is still applicable with the life lesson: Just because you have access to superior tech doesn't mean you need to use it 24/7. I have access to advanced dropsuits, prototype/advanced modules, prototype weapons, and prototype equipment. Want to know what I usually use?

Standard/Militia

You're probably scratching your head right now, but let me share something with you I've learned from observation:
I have a friend that I regularly play with that has access to advanced gear and prototype weapons. Very skilled player with a good gun game, situational awareness, teamwork and all that good stuff. But when he dies, all you'll hear for the remainder of the match is how much his fitting cost and how he's not going to get any money from this match if he dies again, he's going to probably go negative, and how did he die to someone using standard gear.

To sum it up: He spends more time worrying about losing his expensive gear than making the most out of it.

My day to day fittings cost probably 1/10th of what he uses. I'm just as efficient if not more because I try to do more with less. Occasionally, I'll switch to my absolute best fittings an play lights out! Not just because the superior tech is making be a better player, but because I'm not worried about what I'm going to lose, I'm focused on the battle.

If you're bringing out your Sagaris/Surya and getting popped too often, downgrade to your Gonlogi/Madruger, or even your Sica/Soma for a while. Once you feel more comfortable, upgrade again and notice how better your experience gets.

To sum things up, here's a popular saying from EVE Online:

If you can't afford to lose it, then don't use it!