Not sure what the stats are, but I doubt that’s true anymore. It was true 10 or 15 years ago – a studio’s game engine was part of its “secret sauce” which made their games stand out. But as with most technologies, game engines evolved and commoditized. Now Unreal is as sophisticated as most in-house engines, and it makes much more sense for smaller studios: they don’t need to sink money into developing an engine, and they don’t need to sink money into training new artists – they already know Unreal. (Same applies to Unity)
I think it’s mostly larger, and higher-profile studios which use in-house engines these days. Pretty sure Ubisoft has one. Insomniac probably does – they’ve always had a big engine team. EA probably does. Probably pieces of Activision/Microsoft/whatever.
There are plenty of prominent titles on Unreal and Unity these days. “Returnal,” for example, is Unreal-based.
We do use 3rd party assets at times; but it’s a slippery slope. Effects aren’t engines – they’re not that difficult to develop. You might save a little time using one; but, you open yourself up to bugs. So, does your money go into development or QA? It’s a tradeoff.
My current project has some 3rd party assets, but I’m pretty sure we’ll phase them out/redo them ourselves over time.
We use Houdini for lots of stuff. But Houdini doesn’t have a runtime component – it’s strictly for creating and editing assets. A typical use, for example, is rendering texture flipbooks for things like fluid sims, and then importing those textures into Unreal and using them in Niagara. But there are more sophisticated cases. SideFX has a whole showcase on their website.
Well, there’s a question…
Yes, they’re primarily by and for hobbyists/indie devs. But since they’re ubiquitous, they’ve kinda taken over. So, I, as a professional, have to watch them, too (which I find endlessly frustrating). Epic, for example, has very minimal documentation/training videos given the complexity of their engine . That’s why they support the hobbyist culture – free documentation!
So, they shouldn’t be for professionals; but they are, these days. (By my opinion is a little biased – I’m old. Give me text over video any time. )