Hi.
No experience about the WM12, but I've had a (pre Fender

) WM10 for about 10 years now.
Cute little amp that'll get the job done in most situations if loud and deep is not expected. Or, if there's PA support and the WM acts just as a monitor. I have used mine with even my F#BEADG 6er slab on gigs for monitoring purposes with good results, on a stand obviously.
There's a few things about the WM10 that's apparently different from the other WM combos. It (mine anyway) uses a TDA hybrid amplifier chip (not discretes as the rest), which had some teething problems. The later chips are better and the swap is easy since most of the "power amp" is located on that chip.
I bought mine non-functioning, and the DIY repair was ~15€ for the parts, the PS caps included.
Whether that using hybrids instead of discretes topology is a bad thing or not, depends on the view. Replacing discretes is more time consuming and more expensive, the discretes have to be biased, the hybrid does not etc.
The original speaker wasn't to my liking (tired?) and I swapped it for a Hartke XL. It's been said repeatedly in WM10 threads that the Eminence Deltalite II is a huge improvement on both the sound and the weight, I haven't tried that yet even though I have a few on the shelve. Im more of a tube or DI kinda guy, and the Deltalites are reserved for another project

.
The WM10 combo isn't exactly small, but I'd still take that -and perhaps an extension cab for louder gigs- over a WM12 or WM15.
On louder gigs the traditional SWR tone-stack may prove inadequate for feedback prevention, at least compared with the graphic and parametric EQ's found on more modern amps, but the same goes for all "classic" amps.
Regards
Sam