I think a lot of people have the wrong idea on Chesnaught sets. SubSeed, BU, and SD all seem like really poor sets in OU. The only set I would use on Chesnaught is a set utilizing Spikes, which actually seems pretty decent for reasons I will explain later.
Now I don't see the boosting sets being very good on Chesnaught because it's very easy to stop a Chesnaught sweep with a bunch of different Pokemon. It is very vulnerable on the special side and has many weaknesses. Do not get me wrong -- the typing is pretty good defensively -- but selectively good. Chesnaught has very nice physical bulk and resistances, but it's also easy to hit Chesnaught hard thanks to its low Special Defense and weaknesses to many common types. In essence, Chesnaught can deal with certain Pokemon very well, but is also very easily beaten by certain Pokemon.
For SubSeed, the case is a bit different, although part of the reasoning why the boosting sets are poor applies here. In BW, SubSeed was not a successful strategy in most formats. Previously fantastic SubSeed Pokemon like Sceptile became much less good at SubSeeding in general (1st set on DP analysis -> last set of BW analysis) because of the generally fast-paced nature of the metagame in general. The only viable Pokemon with SubSeed are the ones who have little to no issues with opposing Grass-types, as well as either high Speed, Harvest, Prankster (note that Whimsicott has issues doing it effectively because it cannot touch Grass-types without the unreliable Hurricane), and certain other abilities.
Applying BW standards, Chesnaught does not fit them at all. It has a few things going for it with a SubSeed moveset - Spiky Shield, a defensive typing that allows Chesnaught to set up Substitute pretty easily, and the ability to break past Ferrothorn - but it's not enough to warrant using SubSeed over Spikes.
Anyways, I think Spikes / Synthesis / Seed Bomb / Hammer Arm is nice on Chesnaught because it really takes advantage of Chesnaught's attributes: it can counter certain Pokemon (such as Tyranitar, Excadrill, Landorus-T, and others) very well, but is also easily beaten by other Pokemon. Chesnaught's ability to counter those Pokemon really well gives it opportunities to Spike as the opponent switches into one of the Pokemon that easily beats Chesnaught. Chesnaught can repeatedly do this throughout the match by maintaining its health through Synthesis because it can get quite a few easy switch-ins. Meanwhile, you are (hopefully) weakening the other team with Spikes damage.
This is just a different perspective. You obviously do not have to agree with me on everything, but I feel like a lot of you guys are missing out by using sets that do not have Spikes on Chesnaught. I will probably try the SubSeed set at some point because it seems half-decent, but SubSeed as a strategy in general has not proven to be effective in the last generation.