You are aware that Ubuntu ISOs can be written with various ISO writers that re-format the ISO such that it will only boot on uEFI or legacy systems (but not both). Thus the software used to write ISO, plus options use really matter.
I QA-test using mostly older hardware (pre-uEFI) and have had no issues, but I write the ISOs using the documented procedures (clone only) such that my thumb-drives will boot on all devices (be they using legacy or CSM, uEFI or Secure-uEFI).
There are also some boxes that have unfixed firmware bugs that cause an ISO to boot very slowly (on releases >20.10 especially), with some taking 11 minutes with ~8+ minutes showing either nothing on screen, a blinking or just static cursor (box firmware specific as to what shows). Did you wait long enough? It's very easy to assume a failure with 8 minutes of no-output. Yes you can write the ISO so it boots faster on those devices, but it'll also fail to boot on other devices.
With details you provided, you've not ruled out the issue being caused by ISO write method (did you clone it to thumb-drive or reformat ISO?), you didn't wait long enough, or something else (including there is a problem).
Being specific as to the ISO is also helpful; if you gave an ISO date & I have it, or could download it, I can test it on ~8 pre-uEFI boxes for example. If you're not familiar with the dates used on ISOs, the current ISO QA test site (http://iso.qa.ubuntu.com/qatracker/milestones/446/builds) shows the VERSION (date) detail to the right, ie. currently it's 20230928 (YYYYMMDD format) for Ubuntu-MATE. That detail can also be found on the system itself if running (*top of the /etc/apt/sources.list file etc) The date is also seen at download time too (date being related to creation time, not when you downloaded it)
I've not noted any issues booting 23.10 ISOs on legacy or pre-uEFI hardware (my oldest test box being from 2005 though RAM & CPU upgraded since then)