I think that Spiders vs JJ is balanced if the JJ player can just get past the Flea harassment without getting too far behind but that is highly map dependent. If there are too many mexes and they are too far apart like on Trojan Hills or WanderLust, JJ simply cannot contest with Flea harassment while also sufficiently preventing the spider player from expanding nakedly. Fleas are also a decent defensive counter against lone Pyros if you micro them correctly even if the Pyro uses jump to escape the range of Fleas for a short while.
The fact that a single Flea can force a Constable to retreat makes JJ expansion prohibitively expensive against Flea harassment. Even if the Constable can retreat into the range of defenses, the Fleas can just sit outside the defensive ranges and the JJ player is forced to either defense creep towards the mexes or bring a 220 cost Pyro to deal with a few Fleas.
If the Constable could kill a Flea in 2-3 shots I feel that the match-up would be better balanced and 7-10 DPS would hardly matter in other match-ups.
However, I do think that JJ is potentially sleeper OP due to their high micro units and high skill ceiling.
-Constable, the cheapest con that is fast, can jump and has a slowing beam. Totally OP with the correct micro but it is relatively weak and often automatically targeted with high priority by enemy units so using it offensively has to be done with good micro and care if they aren't simply to just be wasted.
-Puppy, requires the right targets and will die to raiders pretty easily if it gets kited. Pretty good against Cloakbots(Scythes especially), gunships and airplanes in general, and can be used to suck up contested wrecks.
-Pyro, the difference between a well timed jump and no jump can be huge. Also generally needs to be repaired to make cost against other raiders.
-Moderator, you need to pick targets manually quite often to avoid wasting shots on low priority targets such as raiders, nanoframes, solars and mexes. Being a perfectly accurate skirmishers they can also benefit hugely from manual dodging and being retreated for repairs against other skirmishers like Ronins and Rogues. If caught out of position, they are easily overwhelmed by cheap units so even their maneuvers have to be premeditated with risk-assessment of being overrun.
-Placeholder, often requires manual micro to be effective or it can even cause harm to your own units.
-Jack, super short range and being relative slow means that you have to box in the enemy to deal effective damage. Also requires heal to be effective.
-Firewalker, great unit against Spiders, Shieldbots and Cloakies. Turning too quickly for their weapons to keep up, it is not rare to see them dancing around at the frontline without firing against moving targets. They easily waste their shots on lone enemy units and to protect them against surprise groups of raiders, Ravens, Sharpshooters, Widows and Scythes their employment needs to be premeditated and distinct. However with a few puppies running circles around them, a few Pyros guarding their flanks, a field of literal fire laid infront of them and some flanking puppies or defences to limit the angles of approach for sharpshooters, they are almost impossible to destroy cost effectively by said factories.
-Jugglenaut, dies easily without proper support. Needs to constantly take damage when pulling in enemies and be repaired at some point. Sometimes you have to constantly juggle pushing and pulling to be effective and not draw in too many enemy riots that kills your reloading Moderators. Jugglenauts also makes using Firewalkers and Placeholders harder as they will fire at units pulled into the Jugglenaut.