Kim, I have had some experiences with Android GPSs that many folks call an outlier. With my own experience developing mission critical systems, I don't believe it's nearly as rare as people here think. Even if it's broken (which I doubt).
There are GPSs, and then there are GPSs that work. It's not easy to know which yours lies in, especially if it's in a phone or tablet.
My worst example was when I transited Class B from WVI enroute to LLR. The tablet lost a signal somewhere around HAF and showed my position as though it were current, northwest of OSI VOR outside of Class B. Fortunately, I'm very skeptical of splashy looking devices and transited the airspace using pilotage. And I knew I was approaching Stinson Beach, just about to come out of Class B on the other side, and had done so with appropriate clearances. That's more than a 20 mile error, presented as though it were perfect.
Had I used that tablet for airspace avoidance, I'd be defending my certificate now. Is it an outlier? Tablet GPSs frequently have problems holding signal inside a metal cocoon. Try putting one on your car seat and see how it does. The thing I object to is that this thing presents a lost signal as though it was not lost. From an engineering perspective, that's a fatal flaw.
KSFO Class B is set up to identify boundaries using VOR radials (mostly from SFO VOR, but the north boundary is an exception, using SAU VOR) and DME. But the "right" way to do this is with visual landmarks on the ground. It's not at all difficult to do this, and that's how you avoid Class B airspace. Even at night, this isn't very difficult. The terrain is more of a challenge, as many of the peaks are not lit. And the last thing you want is your head down at night, as that's just asking for spatial disorientation. It's really dark west of the mountains.
When transitioning from the north, ask NorCal for a transition as soon as you get handed off. The boundary is along a line roughly from Angel Island to the Carquinez Bridge. If you think you're too close, descend to 2000 and you can delay contact until over the City (but Oakland Center can be easily cajoled into a somewhat early handoff for this purpose). There are a TON of good (and lit) landmarks around there. From the south, the boundary is roughly Hwy 92, or Half Moon Bay Airport, below 4000 feet.
I've said this many times. It's easier to get cleared through Class B than it is to avoid it. I don't know if you've tried it yet, but you really should if you haven't. Typically, NorCal tells you to remain west of 101 at 3500. If transitioning west of the mountains, they will want you to stay west, and at a higher altitude. During the few times NorCal hasn't let me through Class B, they did take me through OAK Class C, with step by step instructions how to do so (in a nutshell -- Coliseum at or below 2000, 29 numbers, mid-span San Mateo Bridge below 1500). These guys are VERY helpful.
Trying to avoid using this service with a tablet is placing unnecessary risks on your certificate. The tablet isn't necessarily helpful for this. More importantly, you can't know for sure when it is being helpful and when it isn't.
What it CAN help is preflight planning. There are several planning apps -- like Foreflight, fltplan.com, duat, etc. I rejected Avilution because it rendered the SFO TAC with an aliasing error, and was quite difficult to read. Garmin Pilot is more readable. But they are still working out the crashes (it's a lot better than it used to be). Quality is highly variable; Garmin Pilot suffers the usual poor interface design that every Garmin product does. But a tablet or phone can be useful for logging, flight planning, filing, weight & balance, and several other things. I just don't agree that airborne navigation is one of them.
As a pilot, I'll suggest that the GPS feature is not a good thing to rely upon. If you ever feel you need it to get around, you're in a "bad place" and should get some time with an instructor or maybe another pilot to resolve that.
