I own an Onkyo PR-SC5507 which is very old now but was top of their Processor range at the time. It has has 2 new HDMI boards and has recently died for the 3rd time (hence I am looking around myself). I don't know if they have fixed their quality issues but you may wish to do a search on Onkyo and dead HDMI boards.
My vintage Onkyo SR876 died of HDMI board failure (it continued working via SPDIF and all non HDMI inputs!
My subsequent Integra DTR 70.4 (roughly same vintage as your 5507) - died of DSP board failure, also a well documented manufacturing fault, not of Onkyo, but of TI the maker of the DSP chips.... no replacement boards are available anywhere within the Onkyo/Integra chain globally.... (they just got back to me a couple of days back)
Having said that.... Onkyo had a spate of issues from around 2008 to circa 2014 - first a batch of under-rated capacitors... which caused the HDMI board failures, then a bad batch of DSP chips.... in both cases, the issues were exposed by excess heat. Those AVR/AVP's all ran seriously hot on the processor chips (NOT the power amps!). - The heat caused the already marginal capacitors to fail, resulting in free HDMI board replacements.... unless your exemplar like mine, kept working until the replacement program expired (I gave my AVR lots of air space, and at one point experimented with fans) - the price I was quoted for a replacement HDMI board, led to my purchasing a refurb Integra instead....
That integra (and probably your SC5507) used later DSP chips, sourced from TI, a large production batch of these ended up with a documented manufacturing fault, where the DSP chip would warp/twist due to heat... which would disconnect some of the ball grid array connections on its underside.... killing the AVR. Again - if the heat was adequately managed, if there had been a better heatsink (or ANY heatsink?) on the chip, the fault might not have manifested. - Replacement boards were made available for some years.... my well cared for example, with loads of airspace, failed more than 2 years after the replacement program closed....
But yes, I went back to Onkyo/Integra again... Why? - well first of all, in terms of features and performance per $ - they are the best value on the market.
I was seriously considering Denon and Anthem as alternatives, but all my research showed that models from 2016 onwards no longer had the heat issues of the previous generations, reports on post 2016 models were consistently positive in reliability terms over the long haul.
I dipped my toe in with an Integra DRX3.4 - sufficient channels for my purpose, and its onboard power amps were of no major relevance as I am using my external amps anyway... (I am hoping that they will release the flagship models soon.... as I would seriously consider those, to simplify my setup... might not need the external amps! - and also for Dirac ART)
With how cool the current generation runs, I have hope that its reliable lifetime will be far greater than its predecessors (you can barely feel a slight warmth at the rear above the HDMI board). The Onkyo/Pioneer/Integra's run substantially cooler than their DenonMarantz competitors (apparently... My example certainly runs remarkably cool... others have had both and compared).... and with any sort of complex circuitry of this type, how cool it runs is a long term indicator of how long it is likely to last!
So yep - I'm giving them another go.