[buzzybee]

Garbage collection is troublesome for real-time processing(including game engines) because it doesn't allow you to plan around hitting a latency deadline. When the collector triggers, it consumes a lot of time, which can add up to missing a deadline even in very relaxed situations.

Historically, many game engines have a "never allocate" policy: Everything is done with static buffers and arenas of temporary data. The broad strokes of this policy are mostly achievable in a GC language that allows value type collections(fewer managed references = less to trace = cheaper garbage). The problem typically comes in little bits of algorithmic code that need their own allocation: Because the language is garbage collected, all your algorithms are using the GC by default. And if you want to reclaim it, you have to fight a very uphill battle.

IME, though, most of the problem is resolvable if you have enough introspection into GC configuration. If a game can tell the collector that it needs a cheap scratch buffer to process an update loop and then get thrown out, that covers a lot of the problem. The last bit of it is fine detail over memory size and alignment, which some GC languages do give you introspection into already.

Edit: Also, I should note that the relative value of GC changes a lot when your process is long-lived and unbounded in scale(servers) or involves a lot of "compiler-like" behaviors(transformations over a large, complex data graph). The advantage of doing without it in a game engine has a lot to do with the game being able to be tuned around simple processing of previously authored data, with limited bounds in all directions.