Stealth builds are designed to keep the ship at a heat level that is undetectable by enemy radar, either for the opening salvo, and/or for a high amount of the ensuring firefight. Both types allow players to approach targets unaware and get an upper hand from the start of the engagement, tilting the resulting fight in their favor. The latter then continues the fight with the opponent unable to utilise their radar, which is necessary for gimbals, turrets and missiles to function, and also makes aiming fixed projectile weapons substantially more difficult due to the lack of a predictive reticle, not to mention you need to eyeball the target perpetually as you can't use the radar to determine their position.
I personally hate cheap tactics that give an opponent an unfair advantage, but I'm also aware that that position doesn't constitute the title of exploit. An exploit is, by definition, using a system in a way other than intended. Given FDev has applied multiple patches (some of which break basic mechanics, like turrets not needing a lock to return fire and CMDRs showing up on radar regardless of heat once they open fire), not to mention implementing emissive exclusively to counter heatsink spam, it's safe to say that players being invisible to radar throughout a firefight is not something FDev intended when they designed the combat aspect of the game, therefore, it's an exploit. Despite this, the stealth opener is still on the table, it's not until you're in the fight that emissive becomes relevant, and as FDev have done nothing to prevent stealth openers, it doesn't meet the title of exploit, but it's certainly in the same realm, hence, "borderline exploit".
Nerfing emissive isn't going to bring back stealth builds, FDev have nerfed them into oblivion in more ways than one. But, in my opinion, we don't need to be giving stealth PvP a buff, even if it's a minor one. Cheapshotting should stay dead. Then again, I'm hardly the most experienced PvPer, and thought weighing in in a more comprehensive manner would be above my pay grade, so I made my earlier comment short.