It lacks Multiple Inheritance and doesn't provide an alternative. This sucks.
After several days trying to grok what the hell happens to cause it to generate a query with an aliased table missing (i.e. attempting to select t3.foo without specifying what t3 was).
Finally today I discovered that when I removed a layer of inheritance it suddenly worked. so if class A inherited from Class Super, then you can insert objects of Class A nicely into Class C via a relationship and it won't barf horribly. However if you make Class Super inherit from Class SuperSuper then the insert/fetch/etc calls will barf as Tangram gets itself all confused with no hint as to what the problem could be.
In fact the more I use Tangram, the more I think "given a month I could do a shit load better than this", so much so I reckon we (myself, a web designer who has picked up some perl, and our sysadmin) could replace the PoS pretty quickly if we sat down and coded something that meets our needs without trying to be clever and pure and 'elegant'.