Most of the issues are those common to all 'unofficial' ROM updates:
Make sure you know exactly what you are doing, because there will be no warranty support if things go wrong.
Be prepared to write off the device if things do go badly wrong with the hardware, and you can't reapply the original Orange ROM (thus being unable to ask for a warranty repair- thougbh some users have reported successful warranty claims with modded ROMs)
Prepare the update well- apply any SIM/CID unlockers, and Hard/Soft SPL bootloaders before trying to update the ROM
Research what you need- there are now about 9 or 10 WM6 ROMs for the Trinity; check all the relevant threads on XDA Devs
Back up you essential data to a PC, and have all your registration codes to hand for applications- you can't restore a backup set after a ROM update (despite what Sprite etc may tell you- most people trying this have big problems)
I've tried a number of the different WM6 ROMs on my M700, and in general they are all pretty good. My preference is for the Mun-Rus versions, especially the new Indigo Lite, which gives 60MB free storage memory and is blindingly fast, as it omits a lot of the unnecessary stuff (for me at least- I don't use Instant Messaging, Transcriber etc, etc)
A word of warning if you use TomTom Navigator. Some of the new ROMs are based on the DOPOD WM6 ROM. This has a bug in it which changes the Device Code after every hard reset. TomTom will not issue extra Activation Codes to account for this as it breaches their licensing policy. Indigo Lite is affected, but apparently Indigo Full is not, as it uses the very latest DOPOD ROM , which has been fixed. There are reports however, that Indigo Full causes some problems with WiFi.
Mun Rus TE and Indigo Lite have both worked well for me. The WiFi has no progblems. GPS is not affected by these ROM updates, because GPS is a part of the Radio ROM, which is not updated with these ROMs. As long as you have a GPS enabled Radio ROM (1.30.00.11 onwards) all should be fine with the GPS. Radio 1.38 still seems the most stable.