I agree with this. As someone who uses Unity without issues (unless we update and some systems move or are deprecated) - its a very GOOD engine if you use it within its means. Garbage collection is my biggest beef but you can work with it with some optimization.
Now - to the issues.
-Its an all-in-one type of solution. Its not amazing at any one thing, rather, good at a whole bunch of stuff. Devs need to keep this in mind. It can't do everything and is poorly suited for several jobs.
-Extensibility through asset packages sounds good on paper, but again they are developed by third parties with the constraints of Unity and live in the same box. If the engine isn't capable of doing X - creating an extension or asset that tries to do X isn't going to be 100% flawless because that asset lives inside of Unity's constraints.
-Updating extensions sucks - you are at the discretion of the developer of "SUPER AWESOME CONTROLLER INPUT ASSET" or "STREAM ANYTHING ANYTIME FOEVER AND EVER ASSET" to update their software to work for whatever hardware you need. There is no guarantee that the developer of the extension will be around, will ever update, etc
-Garbage collection is very much so abused due to it being automatic. When you make a big game damn near everything should be pooled. Design the nuances of your game with this in mind and you will be one step ahead. Don't and you will have hiccups as memory is freed to make room for more streaming.
-Manually calling that garbage collection is typical of streaming extensions and assets. Its what causes the most stutters, quick pauses, hiccups, etc.
It's a very workable engine if you use it within its constraints and then some. The minute you try to push against its walls it begins to shake a bit. You CAN and you can do it without issues but it usually requires you to roll your own middleware for what YOU need, not something else that was designed as a "catch all" solution. Case in point: I don't like Unity's particle system so I wrote my own that works the way I need it to without bloat and is built with Unity's constraints. Do I have issues? Nope. Did it take time? Yep.
Its very easy to try and take things beyond what an engine can do. Its very capable when working within its constraints and becomes unruly when you try to move past them.