[Martial Artists Fight] Goku is a decent unit, he can stun and even tank pretty well after launching the Super Attack; the damage output is nothing amazing, but it's still pretty good, and the unit does have some interesting links that can help him fit on a Super PHY team.
Pros
Good tank Decent damage Stunner
Cons
Lacks common ATK links Low stat boost (B Rank Potential System)
Supreme damage is nothing special, but at least the unit has other ways to increase its damage potential and the 12 Ki multiplier is 140%, which is pretty good; the Super Attack has a 30% chance to stun that makes this unit pretty good in terms of utility.
Leader Skill
No boost to HP and only 2 Ki, but 60% to ATK and DEF is not that bad for as Sub-Leader SKill for Dokkan Battlefield, if you lack units.
Passive Skill
Getting an 80% boost to ATK and DEF when performing a Super Attack is sort of double-edged sword: the unit will not be able to tank before launching the Super Attack, but, since Goku is going to be a floater, he will only be attacked after he receives the boost; the unit will receive a much greater boost if support is on rotation, but, again, if Goku is a floater, he will likely not benefit from this.
How good is this Passive Skill?
In a realistic scenario, Goku will be in the last slot and no support will be on rotation, meaning that at 55% in the Potential System on a Super PHY team, his DEF will be at 20.379 before the Super Attack and at 36.683 after it, which is already a very good value. Now if we want to create an ideal scenario, we would have to put him on rotation with the support Gohan I mentioned before; this time his DEF will be at 32.811 before the Super Attack and at 59.060 after; this amount of DEF is more than enough to qualify him as a tank, he will probably take less than 20k damage from normal attack in Super Battle Road, which is quite a feat for any defensive unit.
Link Skills
Shocking Speed and Shattering the Limit are pretty good Ki links, but the unit would have probably preferred having Prepared for Battle since so far the only competitive team he's on is Super PHY, where that is way more common; Infighter is quite nice, it can increase [Limitless Radiance] Super Vegito's ATk, but if Super Vegito is in the first slot, blocking and countering most of the attacks, and Goku is in the last, it won't matter. All in the Family is not that easy to activate, but, as you saw in the example of the "ideal scenario" I made before, it can be activated and it makes quite a difference since it's a 15% DEF link; Kamehameha is probably going to get an update in the future and, even though it's that that common in the element, it would be nice to activate it for [Fusion of Two Powers] SS Goku (Angel) & SS Vegeta (Angel).
Gohan (Teen) doesn't share any Ki links with Goku, but his support Passive makes up for it providing him with 3 Ki and a 40% ATK and DEF boost; in addition to this, Gohan (Teen) activates "All in the Family" for an additional 15% DEF, giving Goku a third multiplier.
Super Saiyan God Goku is a pretty good support unit, he is going to give Goku 2 Ki and a 25% ATK boost with his Passive while also activating both “All in the Family” and “Kamehameha”.
Goku (Kaioken) and Goku share both “Shocking Speed” and “Shattering the Limit” for 4 Ki and, by linking together, they can activate “All in the Family” and “Kamehameha” for each other.
Pan (GT) does not share any Ki link with Goku and her Passive doesn’t add anything in that department, but she can activate “All in the Family” for a 15% DEF boost and “Kamehameha” for an additional 2500 flat ATK; Pan (GT)’s Passive gives all allies a 30% ATK and DEF boost, partially making up for Goku’s overall lack of ATK links and further enhancing his defensive capabilities.
Great Saiyaman 2 is a very good unit that is also pretty easy to obtain; she is going to give Goku a 40% DEF boost with her Passive and also activate “Infighter” for a 10% ATK boost.
Hidden Potential
Additional Attacks are nice to raise the chance of stunning the opponent, but Criticals will always be the best way to go in order to maximize the damage potential; the unit has a B Rank Potential System, meaning the average stat boost at 100% is 3000 instead of 5000, thus I would not advise putting orbs into this unit as the value you will get from doing so is not much.
Should I use Elder Kais/Grand Kais?
Fortunately enough this unit has a farmable Super Attack; you can use Long-awaited Fight Goku from the Dragon Ball Super: Universe Survival Saga Tournament of Power Story Event to raise his Super Attack. Would this unit be worth investing Kais in hadn't the Super Attack had been farmable? Definitely not, there are a lot of PHY units that would deserve them more.