Hi Developers,
I’d like to know your opinion about Roblox Studio solid modeling. Are unions good to use? I’m asking because i see many people that use unions and some people that say “unions are sux”. Now I don’t know whether to use them or not.
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They aren’t ideal at all. They can lag your game, you can’t change the material of unions, and they are buggy.
@iGottic can explain more. 
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It doesn’t take a lot of experience to do basic modeling in blender, you can even build in blender. For example, you can build up to x5 faster than a builder when in blender. It has a lot of dedicated keybinds for it and you can just use multiple objects for parts and I’ve heard they decrease lag. Not sure if that last thing is true but all in all I think eventually builders will have to switch to modeling to keep up.
Unions can be really good and useful. But sometimes they have some bugs. For example, there’s some times that I try to make a hole on the ground. But when I test the game and I try to get inside the hole I can’t. It’s like… there’s a transparent part and you can’t go through it and not a hole. It annoys me but well… hope I find I way to fix it. I still don’t know if it’s my fault or Roblox’s fault.
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you can go into properties and enable precise collisions to go inside the hole
this applies to both unions and meshes
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I have been summoned 
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I know is marked as solved but still, I want to give my opinion.
Unions aren’t bad, but too many of them could actually cause a game to lag. You could do a big union, or transform it into a big mesh, if you want to prevent lag and make it work.
The main diference with this both, is that you could make an entire map city to become a mesh, to reduce lag, as its a single piece, it should reduce it.
Unions do sort of the same, but I’d personally suggest to use unions for small modelling or creations, and transform them into meshes for big creations.
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You can change the material of unions, though.
But you can also change part materials!
change your collisionfidelity property on your union to precise and that should fix it (it’s not perfect but it works well, costlier performance wise though.)
as for OP, please refer to this guide: Quick Guide Into CSG: Increasing Performance of Unions & Meshes
I will copy/paste what I said in another thread:
To surmise, are you going to re-use your unions a fair bit? - may be better than just having those models be parts (eg, if you have a chair unioned and re-use that chair as opposed to having the chair remain parts.)
are you going to union for the sake of reducing parts, but you aren’t going to re-use said unions? - bad idea, creates unnecessary triangles and collision issues.
are you using unions because you have no other options? - change colissionfidelity to box if the player will not interact with it.
meshes/unions are instanced about the same, but unions are more costlier in terms of tri-count which is why (generally) you should not have many unique unions if it’s not necessary. You cannot really control tri-count in Roblox. If you have one union, copy/paste that and re-use it, it’s good for performance rather than having several unique unions.
unioning several transparent parts that are close together is not a bad idea either.
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