The Rosat (and its late introduction)

[Captain Cadaver]

It goes without saying that the Rosat was fairly contrived in how it was brought into the plot with no prior hints to it and questions such as why it wasn't used against for training to face the Saiyans or the Artificial Humans during the training timeskips. It makes sense to not bother with it for the Saiyans when it's unlikely any of the Earthlings could handle it back then, but what about the 3 year training?

I'd probably say the best explanation would be that Goku wished to keep it as a last resort only necessary when his current training and level of SSJ weren't enough due to the intensity it provided even to the Saiyans after those 3 years of training (not to mention its 48 hour limit and it taking 1-2 years off one's life). Piccolo probably didn't become aware of it until merging with Kami either, who would've been very sceptical on anyone lasting a year in the room.

Also, is there any explanation that can be given to smooth over the obvious plot hole in the Boo Arc of the 2 person limit being contradicted (without resorting to supplementary material such as Kakarot, of course)?

 

[SSJ2]

I genuinely can't think of a good reason. I agree that the earthlings probably couldn't handle it in preparation for the Saiyans, but there's no excuse during the 3 year gap. Goku and Piccolo made very good improvements on their own during that span, but we saw that they made even more enormous gains after a year in the room. Imagine if they both spent a year in there at the tail end of the 3 year gap.. Cell never would have gotten past his first form.

Also what did Kakarot say about the 2 person limit?

 

[Captain Cadaver]

Goku and Piccolo made very good improvements on their own during that span

I'd say that in itself offers some rationale for not making the room the first option. It is pretty much a last resort when it can only be used fully twice (prior to Super just treating it as an instant plot hax) and takes off a year of one's life, so I can see why Galu wouldn't want to use it unless he was absolutely sure his earthly training and the SSJ form couldn't overcome an obstacle. He didn't have a complete idea of how strong the Artificial Humans were beyond much stronger than Trunks, after all.

Also what did Kakarot say about the 2 person limit?

There's a substory during the Cell/Boo Arc Intermission in which Gohan helps Dende upgrade the room so as to allow for more than 2 people to use it at a time.

 

[SSJ2]

Goku and Piccolo made very good improvements on their own during that span

I'd say that in itself offers some rationale for not making the room the first option. It is pretty much a last resort when it can only be used fully twice (prior to Super just treating it as an instant plot hax) and takes off a year of one's life, so I can see why Galu wouldn't want to use it unless he was absolutely sure his earthly training and the SSJ form couldn't overcome an obstacle. He didn't have a complete idea of how strong the Artificial Humans were beyond much stronger than Trunks, after all.

I don't think it should have been their first option, but they should have used it near the end of their training. Piccolo wasn't completely confident that the outcome would go their way despite all of the training they did, and he ended up being right. It would have made sense to use the room if he wasn't fully confident.

 

[Hector]

Wasn't it stated they were afraid they wouldn't be able to handle the conditions before Goku encouraged them?

 

[Captain Cadaver]

I don't think it should have been their first option, but they should have used it near the end of their training. Piccolo wasn't completely confident that the outcome would go their way despite all of the training they did, and he ended up being right. It would have made sense to use the room if he wasn't fully confident.

It's questionable if Piccolo knew about the room's existence prior to his merging with Kami, not to mention he was confident in terms of power and only had a strange foreboding feeling, something he only revealed as they were going to the battle and which Goku's answer to probably wouldn't be harsher training.

Wasn't it stated they were afraid they wouldn't be able to handle the conditions before Goku encouraged them?

Not directly stated, though somewhat implied by Piccolo's reaction to Goku bringing it up.