I would say unions are reliable but if I would like to build something high-detailed I would go straight into Blender. However unions do corrupt sometimes as some people have said already.
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Unions are useful when you want to make small objects of the same material, plus they are quite reliable. I use unions for details on buildings, or stuff like furniture, it just makes it feel cleaner for me without the overlapping parts.
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Unions are a great finishing touch for adding windows and doorways etc however this should only be done once you’ve finished everything due to measurements getting messed up by unions.
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The primary problem with unions is that, unlike individual parts, unions are created as an asset and are loaded to the game from the website. If you have too many unions, then you are loading so much data from the website that it will lag the game.
In addition, unions can create extra faces and triangles that you might not need which puts stress on the server. I use unions very sparingly in my newer games.
There was a time where people would union literally everything under the guise that ‘well its less parts so it must be better for performance’. This is obviously completely wrong and infact makes things worse.
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The primary problems of union is that they can corrupt. This has been seen in many, many cases. There is a page in the DevForum explaining how you can recover corrupted files. Unions are only good for small things and I would highly advise not using them in big or detailed structures. Once again, if you really have to use unions, export them and import them as a mesh. I would only recommend using unions if you’re doing tools or doing small and unimportant things. Unions should never be used as a primary resource for advanced parts.
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I use cylinders and spheres in unions all the time. Haven’t tried wedges in a good while, but they also worked fine in CSGv1.
The way I see it, unions should be used in the same way that they are in Unreal Engine 4; as BSP brushes. I only use them to make basic shapes when I am blocking out my scene. 
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I normally covert unions into a mesh instead of using the unions as they’re unreliable
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I’ve been using union for a small item until this day, mostly to fix the texture overlapping. And for me, the corruption event occur rarely. So i guess its fine using it as long as you use it for a simple thing that isn’t hard enough to replicate. I still use grouping on a big model though.
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For me they don’t work, lol the 30 character limit is useless
I sometimes make copies of some of my work before hand in-case the unions fail, unions sometimes work for me, but rarely. They often have bugs and in my cases are not reliable for big projects; I would use them for simple builds that aren’t too significant. You’re probably best going to blender.
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Mmmmm… I say just learn how to use Blender. You’ll be better for it in the long term.
Not only that, but you can use Boolean Operations in Blender as well and with much greater control completely making Unions useless in the aftermath. They’re just not worth it whatsoever.
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Only use unions when absolutely necessary. They are prone to corruption very frequently in my experience and don’t offer very many benefits over the alternatives.
Despite popular belief, unions are NOT always better on performance than a bunch of parts are. The geometry for unions is sloppy compared to what you can do by hand and will almost always have messy collisions and vertices which can lead to unexpected lag in your games. Along with this, the geometry data for roblox parts is already stored on the client so loading in several parts may actually be more efficient than an entire new union in a lot of cases. This issue compounds especially hard when you are using a lot of new unions in your place instead of duplicating existing ones. Going about unions this way can severely increase your load times and resource usage.
Personally I avoid unions wherever possible and usually only use them for objects that are relatively simple and have collisions disabled. If you absolutely must use a complex union, make sure you are duplicating it if you need to use it more than once and you should consider disabling the collisions to improve performance.
Hope this helps answer your question! 
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I use unions to make it more easy to go through a model and a game on the workspace rather than having thousands of parts that say “Part”
Unions cause a lot lag. I personally have only used them once for a macintosh model. After that I saw them as unreliable
For one, unions don’t remove any of the lag that importing a mesh from blender removes. (Let’s say you are making a flower pot, and by unions it’s 70 parts. 70 parts is laggy but you can make it in blender for only one part. With unions Roblox registers it as 70 parts still. Also, there are reports of CSG issues (invisible blocks) that come with unions.
For some things I think using unions are fine. For example, unioning grass blocks to stop z-fighting. For others, like making a lantern, definitely use blender. It’s more reliable, it makes less lag and you’ll get a better product.
If it isnt backed up though, a union is permanently gone/corrupted, plus, to back it up, you’d probs have to upload it as a model or save as a place file which sounds not so good.
Meshes are already created from you meaning you probably already have a save file of it, plus, putting it on automatically keeps it saved and you can adjust it in another program easily with textures and more.
Meshes in my opinion are better in terms of reliability but if you want something quick and fast, unions are you go to thing for speed.
Ah Unions, Definitely a very interesting topics. Unions can be reliable In cases small but big cases like making a building a union is a lot of stress. Most of the time you make the union but then the union could ruin the whole building. Which took most of the work and it is a terrible feeling. So in some cases unions are reliable yes.
Unions are completely reliable and trustworthy.
They can bug out on you sometimes, but that’s the craft of working with them. You figure out what works and what doesn’t. Though that works contrary to what some people may want in terms of freedom, it gives you an excellent look of what you can easily do with unions and what you should probably do with Blender and stuff like that.
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