Pick a high enough number and any system will collapse... 
(I assume it's not possible to make that much money without cheating?)
Define "cheating"
I did skip missions in debug mode, which meant highest score and spending nothing on ammo. Funding countries reached the cap by 2000-2001. (Note: this mod starts in December 1996.) Though I lost 4 of them to infiltration. Plus workshops. I reached 2^31 by mid-2002.
I think this sum is reachable if you don't go to Cydonia, but continue playing long enough and with good results.
As for big numbers, C++ compilers of 1990s had no trouble with integers from -2^31-1 to +2^31. Most modern compilers can handle -2^63-1 to +2^63 if the programmer uses the right types. By the way, so could Borland compilers that were used for the original X-Com games of 1990s.
But a separate bug report thread may be a good idea. I guess the only reason it hasn't been stared yet is 1) most bugs are reported to modders, not directly to Meridian, and 2) there are very few bugs in OXCE+. 
This particular bug is not related to any mod and does not reproduce in vanilla, OXC or OXCE.
Unrelated bug about OXCE+ compilation.
cmake checks that yaml-cpp version should be above or below 0.5.2. But that no longer seems to work — oxce3.5-plus-proto branch refuses to build with 0.5.1.
The condition should be "yaml-cpp version above 0.5.2".
Another unrelated bug. Quick search in inventory may give false positives, especially for single-letter queries. For example find "Bone Club" for "w". Observed on Gentoo Linux with "Extended+-3.9c-61d22e5-2017-10-10-xenial-x86_64" build from lxnt.wtf. See the attached _autobattle_.asav (requires X-Com Files mod).
For the new engine version (3088e4ac6 from 2017-12-12):
Pressing "u" in Battlescape causes a crash. To reproduce, start a "New Battle" and press "u". Observed for "X-COM Files" mod and without any mods. Not applicable to OXC and OXCE, where "u" doesn't work. See "Ufopedia-crash.log".
CRITICAL BUG. When battle ends, unconscious enemies seem to count as dead rather than captured. To reproduce, start a battle without mods and then stun everybody. (Or just press Ctrl-D and Ctrl-J.) — Debriefing lists all stunned enemies as dead.
