Using a Breville 870XL and usually run Umpire Estate espresso from Colorado Coffee Merchants through it with great (to me) results.

I ran out of my favorite Colorado roaster beans and so I picked up some beans from a local roaster with a roast date of about 4 days ago on the bag. I kept my grinder setting at 5 and weighed my dose to ~18g. Minor adjustments in grind or dose weight produce wildly different results but never a good result. Over extraction, way under extraction, and even when I think I've got a good extraction, it just doesn't taste good. I bought two 12oz bags and I feel like I'd save some headache and just flush it down the commode instead of wasting my time grinding and extracting it.

I'd like to think that it's my lack of experience - which could very well be true - but I've had the Breville for ~2 months extracting 4-6 shots a day (I've got an espresso problem) and I've used 3 different types of beans - Umpire Estate, Metropolis Redline, and (eek) Starbucks (one bag, never again) I'm able to get great results out of all of them quickly without wild adjustment changes and with consistent results. Is it possible that I have bad beans?

Yes.

It's possible ... but what constitutes under and over extracted , are you weighing your output , tasting and adjusting your recipe based on what your tasting ?

Keep in mind my limited experience and short time brewing. I'm eagerly learning all I have time for, but this is still a bit new to me.

I'm basing my assumptions primarily on taste and smell. What I have used as a baseline so far on my Breville 870XL when trying new beans:

Start with a medium (5) grind
Weigh dose - 18g seems to be a happy place for most beans using a double basket.
Consistent tamp pressure - I can't guess 30#, but I believe I'm close-ish
Where is the pressure gauge on the machine - I've had best results when it is a 12 o'clock
How does it come out of the portafilter - I like to see it come out closer to 'honey' than liquid - I've had best results there
Crema - much, little ?
Adjust the Program to right at or just over 2oz, which almost always results in a 24-29s extraction

With the Umpire Estate Espresso and the Metropolis RedLine, There are some minor adjustments between the two, and I'll always reprogram if I'm changing beans for the week, but I've gotten great results, wonderful taste, and plenty of crema. I hate to write off a local roaster because of what may be my inexperience, but I've switched beans 3 times and never had this much trouble. The last few extractions I thought were right on, dose was 17g, pressure gauge was good, 2oz'ish in the cup, 24s extraction, bitter coffee. I've gone from 3(fine) to 8(coarse) on the grinder and still same results varying my tamp pressure to ensure the pressure gauge is right.

I've noticed that both of these bags don't have much crema at all compared to Umpire Estate. Bitter acidic espresso isn't my idea of a fun time. I've ordered more from Colorado, I was just hoping I could use a local roaster occasionally. Worst case is I drive up to Memphis again and get some RedLine from Tap&Tamp to get me by (it is good stuff, I just like UE better).

Different beans, different roast levels can require very different tuning. I think your results are inconclusive. If you've tried 25 different brands and had great results with all but this one, then it would be time to write them off.

I would suggest weighing your output to help with your consistency . Until then it's hard to make suggestions based on when you are indicating you think it goes blonde ...