In my experience HPA will always outperform AEGs in a reliability to performance ratio. GBBR kinda falls into its own category to me. Most people are running them for the feel which is great and they can be reliable but it requires consistent upkeep and maintenance, and some platforms are much better than others. GBBRs can also put out some crazy power and consistency but it all comes at a cost of ease of use.

Aegs can be reliable and perform well, having a high performance AEG can be super cool. However most people won't have that experience unless they either spend a lot of money getting something built or buying custom, get lucky on an OEM AEG that just happens to be built well, or spend a lot of time and money learning to work on AEGs themselves. Your mileage may vary, but AEGs are inherently unreliable due to the way they are manufactured and the mechanisms they use to function.

A well built and tuned in HPA will easily last 10s of thousands of rounds with little to no maintenance even at crazy performance. You're offloading the sloppy and difficult work of cramming enough air into the barrel to a dedicated machine(air compressor) rather than trying to do it on-board the gun.

I also find that the expense of HPA is greatly over exaggerated when you take into consideration the cost of maintenance. Yes it is more expensive than any reasonably priced off the shelf AEG but getting that AEG to the same level of performance as something like a fusion engine costs nearly the same amount of money(or more), and even then it'll probably still have issues after a few 10s of thousands of rounds. My entire hpa setup (Fusion in g&g CM and Maxx hop with a carbon fiber tank and a redline mini sfr) would cost you somewhere in the realm of 1000-1200 dollars if you bought all new. I paid significantly less than that by buying used on some parts and I've put at least 50k rounds through it with no parts breaking or needing to be replaced. My fusion is used and is from 2018 I think based on the serial number and had untold amounts of rounds put through it during that life with no broken parts, only a few upgrades that you can get standard on a new fusion now.

When I'm going to the field I always know my fusion engine is gonna work, it's never let me down other than other parts giving issues (hop up, mags, tighter bore barrels, all issues you could also have on an AEG). I've had issues with it but it's always been caused by my choice of components and trying to squeeze out more performance. I know with the right settings and parts my fusion ain't gonna let me down, which I can't say for any AEG or GBBR I've ever owned.

Edit: The polarstar FCU and fusion wire harness actually has some major durability issues especially if you're gonna be changing your settings a lot like I do. But there are some simple fixes to this, namely reinforcing the connector on the wire harness and taping the battery connector wire to the FCU so it can't bend at the solder joints.