(Maybe my problem stems from a tendency to conflate "quality build" with "quality design". I suppose other than the stupidly fragile hinged door over the ports the tray-loaders are at least mostly made out of sturdy materials and reasonably "well built". But the internal design is such a hack. Completely, irredeemably, south-end-of-a-north-facing-mule bu** ugly through and through. And it's not just physical; technically the machines are essentially Beige G3s in drag, with most of the weird glitches and gotchyas of that machine plus a few of their own. They work okay under "classic" Mac OS but once OS X enters the equation they just flat-out suck.
For whatever quality problems the slot-loaders have at least they're a clean design.

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The Slot Loaders are also a hack, they're just nowhere near as obvious. However the internal design is an absolute garbage dump of electronics (no idea why I went for that phrase, but go with it) and the critical structural components holding them all together will crack, flake and fall apart over time. The Tray Loader's materials are of reasonably quality, being much more solid, and the overall structure is sturdier. They have their faults, but at least they hold together.

Compared to the Slot Loader, the Tray Loader is an absolute gem - although they're still incredibly difficult to work on.

I threw away my Graphite 400MHz Slot Loader after the chassis deteriorated. My Graphite 600MHz is headed the same way. I have a brilliant little 333MHz Tray Loader in Strawberry now though. Although I can't say I'm entirely enthusiastic about it, it is in fantastic shape and it works perfectly.