There are a number of reasons that the plan was considered a gamble, chiefly the following:
View attachment 2382
They're on a timer and Boo is destroying the planet while they wait.
When taking into consideration what transpired next, there's more to the gamble than just those factors. It's also worth noting that while environmental destruction was a concern, the time that Babidi gave them (5 days) was multiple days more than the amount of time Goku envisioned for the kids learning fusion (about 2 days or so). Buu's killing spree was influenced and monitored by Babidi - by the time he was killed, Buu still commited violent acts at times, but was also awaiting a potential fight with a strong guy again. He even made a house for himself in the meantime.
Goku classing things as a gamble was delivered in a broad, all-encompassing type of way. And before Gotenks came into being, the most they could go off of is that he'd be better than the collective might of Goten and Trunks; however, to what extent was unknown - so their planning was partially faith-based, making this rather gamble-esque, aligning with how the storyline characterises the circumstance they were in.
Well, this goes past the early stage now. This is after Super Boo, the Boo who is greater than the previous incarnation in every way as per Piccolo's words, is born and makes his way up to the lookout. The narrative has evolved beyond what was established to us during the early stage.
Was he critical of Gotenks? He's critical of his stupidity, yes, and he's pretty cautious otherwise. He never says he can beat Boo outright but he never says he can't either.
Well naturally we're gradually moving past the earlier stage. Each stage is quite symbiotic to each other, but things have been tidy enough so far.
When someone is in need of training, and their is skepticism over their physical abilities, this doesn't sound like someone that reached the goals that were set out for them. That person is Gotenks, with Piccolo making these exact assessments.
Piccolo didn't attribute his lack of development to the change of circumstance, it appeared to be a scolding in which the emphasis was placed on Gotenks.
Piccolo's intent is to have Gotenks train for a day and then fight Boo the following day. Gotenks masters fusion the day before he's supposed to fight Boo and then Piccolo sends the boys to bed after they waste their fusion. When Gotenks goes after Boo after the first successful SSJ fusion attempt, Piccolo's primarily concerned with the fact that their fusion is about to run out, not that they aren't strong enough. I suppose the timer would be the most pertinent factor at hand regardless of how strong Gotenks was, but still -- once that's said and done, Piccolo gives them a few hours of rest before the big fight. There's not much more that can be accomplished in one day.
Piccolo wanted to carry out a thorough inspection of Gotenks' abilities, but this didn't take place due to Gotenks' getting too excited and throwing caution to the wind. Piccolo could barely get a word in before Gotenks took off. There was nothing conclusive about this scene, so it should be understandable to not buy into the one-man hype train here.
Here is Herms's translation of this line:
“Can’t you tell…?! As they are now, even if they perform Fusion they can’t win, no doubt about it…! Tell them that if they don’t want to die while they’re still just little brats, then they should train as much as they can…!”
The Viz wording of "yet" can imply that fusion hasn't been enough all along. This implication is absent in the above line. Something to consider.
Both translations practically fall into the bracket of Gotenks being inadequate. Given that the change in variable (Fat Buu to Super Buu) wasn't credited as the reason for his inadequacy, there's room for the interpretation that Gotenks was simply not in the vicinity of Buu, period.
It should also be mentioned that Super Buu's superiority over Fat Buu was expressed in a very mundane fashion; he was said to be stronger, with a body more suitable for battle. This description only confirms that he has the edge over his previous self. Nothing crazy or jaw-dropping.
Consider all of my points in the section above; wouldn't there be a greater sense of urgency if Gotenks had been behind schedule the entire time? Certainly Piccolo would've tossed the boys into the RoSaT well before this point if he had been, I'd argue, because again, there's not much more that they could accomplish in one day before their planned fight with Boo.
There were multiple things happening simultaneously in a small window of time. Piccolo was focused on observation; eyeing how Buu's behavioural patterns are affected by Hercule, who Buu generally showed his lighter side around. Given how stressful the situation was, this gave Piccolo some time to spectate and be a bit more composed. Metaphorically, think of it like the slow/linear moment of a rollercoaster, prior to the more extreme moment, in which Super Buu came into being and abruptly showed up to where everyone was.
So there was essentially a 180 in terms of the vibe. One that Piccolo or anyone else couldn't have seen coming.
Also, kids will be kids. Goten and Trunks happen to be on the more stubborn side as well.
Thematically, yes. But the characters can't be speaking completely nebulously; their decision-making is still primarily influenced by the concrete situation that's in front of them right now. When Goku presents the plan to fuse, the end goal is to beat Fat Boo with it in two days' time without the use of the RoSaT.
The shift in events gives the situation more nuance.
SSJ Gotenks vs Fat Buu was supposed to be the ultimate fight that would conclude the storyline, whether for better or worse. Since it didn't happen, SSJ Gotenks remained unproven and untested in terms of how he stacks up as an asset against Buu. Given this, and his power mostly being thought of in an idealistic sense within the plot, skepticism is absolutely warranted here.
Fat Buu was depicted as a being that surpassed all imagination. If SSJ Gotenks was stronger, surely that would have been expressed in an undeniable manner? The boys were analysed down to the finger, but this milestone was swept under the rug? Don't believe it.