Kastro was not so bad.
Pro:
He was compared to floor masters (even if we have never seen them, they could be fodder or strong, yet Hisoka hasn't shown any interest in challenging them aka they are not a challenge for Hisoka but of course that is a realm a lot higher of the possible powerlevel of Kastro) and won 9 fights only losing to Hisoka 2 times. Hisoka aalso spared him cause he had hopes for him to develop in something interesting, but in the end that didn't happen.
His powers were pretty cool and indeed managed to trick Hisoka for a bit.
Cons:
He lost his cool so focus vs Hisoka pretty fast when he wasn't controlling the situation anymore.
And when that happened he lost the ability to create his double and that was because that skill required a lot of effort, so much that
a) if you are not calm you can not create your double .
b) and even if calm you can only do that and nothing more .
He overloaded hsi memory with this skill.
Also/and he did a beginner mistake, as remarked by Wing, in creating that double ability cause he was an enacnher and that was a mix of conjuration and manipulation so he focused his all on that ability englecting everything else (this is a bit of a repetition of what Hisoka said but with more usefull/determinant details)
So, imo, his hatsu is a good skill and works but that works until he is cool and also he is proficient only with his hatsu, there is probably not much in the other departments, also we don't know how long he can keep up his clones. If, theory but seems it works with the previous facts, after few minutes he can no longer do it then he is done and that is a big malus. Also the fact that he made that hatsu (manipulation/conjuration) despite being an enancher is a huge mistake that Wing points out to A Gon and Killua was are making their very first steps in the nen field. So CA Gon and Killua can eat him for sure. As for a character about his level... no idea.. maybe between mid tier Greed island guys and Gentsuru? Maybe even Razor? There is also Goto who got done by Hisoka without much effort yet I bet that Goto at least had his nen doing right. His problem is indeed the nentype mistake mainly.