I've encountered a few really painful experiences lately involving nuke assisting. This should be outright removed if possible, or nerfed to a useless level if not.
The problem is that nuke offers a hard limit on how long you have to build an SMD and get a missile up. After you get nuked the first time, for an unassisted nuke, you have to (after losing pretty much all of your base) get an SMD up within one minute. That's not so bad.
But if the attacker makes, say, 15 t3 Hives (or equivalent bp), this means that you have to get up your SMD AND 15 t3 hives up within just 30 seconds (!) if you want to be ready for the next impact.
Clearly it's expensive for the nuker to build a nuke launcher and 15 t3 hives. But for the nukee, it's almost completely impossible to recover from this position because you have to build an SMD and a very large quantity of build power starting from nothing in a tremendously short time period.
This leads to a very bad effect where once the first nuke lands, it's just a permanent cripple. This is especially bad on team games- the team can't really help you, even if they have resources, because you have to focus all the build power in that one spot immediately. If the bases are compressed (e.g. Dual Gap) then you can do this to more than one person simultaneously.
Nukes should not be the permanent area denial mechanism that assisting makes them into. This was nerfed from Vanilla but clearly just not hard enough for the advantages it offers. I see no positive gameplay implications about nuke assisting, and I've experienced many times that it can be a very negative mechanic. Therefore, it should be removed.