I don't have a morticer but I would like to comment on a SIP sliding mitre saw I bought about 5 years ago. I thought SIP was a good make but have some doubts after reading various comments about their products. Again I suppose you get what you pay for and this cost me £135 (cheap when compared to Makita or DeWalt etc). It still works and the fence etc can be adjusted if it should get out of square. However the scale for making bevel cuts was a stick-on affair which lasted about 3 months before becoming unstuck and falling off. The laser guide lasted even less. There is a clamp which I removed because it was always in the way (fouled the saw housing). The main problem however, is dust extraction - I removed the pathetic little bag from the dust port and connected a domestic vacuum cleaner but it spewed out sawdust from the back of the blade. I tried my Axminster 63mm outlet extractor instead but no improvement. I shall rig up a chute to the Axminster and fix to the back, and use the vacuum cleaner on the dust port when I get a chance, but I think this issue is a serious drawback.
So, overall, not brilliantly designed but still going strong after 5 years (occasional use) and a very worthwhile piece of kit. Would I buy it again? Yes, unless of course I could get a better model at a similar or slightly higher price, as I could not justify buying one of the big names.
As a mitre saw, it works and can be set up to be (and stay) accurate despite some disappointing design flaws.
So regarding your morticer - if it has some minor flaws then that is something you might have to live with, having bought a budget priced tool. On the other hand if it can't be adjusted to cut square mortices then it should be returned as defective.
K