I find the key is not to look at the volume of problems but the actual problems; how many are proper software problems, hardware problems, people new to the card not having a clue, people trying to do very strange things (both carts have large numbers or people who like to tinker with things)....
The other side of it is that the acekard is largely supported here on this site while EZ5 stuff is handled over at
http://ezflash.sosuke.com/ (you will probably recognise many of the names from here though), both have quite active IRC channels as well.
Also I am not entirely sure of how many people have what cart (both are easily in the thousands, possibly tens of thousands though, I would like to go by downloads but I know for a fact that any numbers there would be lower than the actual amount by some way) and how vocal each user base is.
Support:
The EZTeam have been around for over 9 years now (absolutely ages in flash cart terms). They still support their older hardware (the GBA slot EZ4 got an update two weeks ago adding support for the latest and greatest, it also has a trickle down effect for the even older carts).
As for the EZ5 they are usually very quick to get fixes out although they are quite often pipped to the post by AKAIO (if only by hours).
Acekard. First off I have to mention AKAIO (
http://wiki.gbatemp.net/wiki/index.php/AKAIO ), it is a third party loader with the main support forum being this site that nearly everyone uses in place of the official loader. It is widely regarded as one of the nicest loaders on all flash carts. The AKAIO team have stated they have no intention of adding real time save and in game text viewer features.
Regarding third parties both teams have released a fair bit of source code but the Acekard teams leads the way here.
Problems.
EZ5i, the main problems right now seem to be with the inbuilt 3 in 1 support (no real use on the DSi seen as it lacks a GBA slot) and the somewhat flaky real time save and text viewer functions (although they are getting better).
Acekard, it has some issues with languages on the DSi which is causing a few headaches (I have not been monitoring the foreign sites for a little while but I hear it is not so good, this is a hardware thing rather than a software thing from what I hear) and more importantly there seems to be a bad batch of sorts going on (the EZ5i has a few issues here but it does not seem to be on as great a scale although again I have no idea how many users each cart has).
Longevity is a hard one, we are all looking at what Nintendo will do next for that one (many of the more knowledgeable types predict an update to block things). If it can be supported rest assured it will be, this goes for both teams.
When it comes down to it if you want to play games, use a few cheats, the occasional soft reset then you will not regret a purchase of either one.