Streamer with built in DAC...is there much difference between brands?

The simple answer is NO, if you are shopping at the same price point but YES if you are moving up the price ladder.  But the orders of magnitude of improvement don't correlate...spending twice as much won't give you double the improvement except possibly the low end where the dac upgrade can be a big factor.  The incremental improvements become less as the prices go up.  I agree with @ghdprentice system cost breakdown.  I think it is a good target to shoot for as one upgrades their rig.

The OP is asking a question similar to “Why would anyone by a Ferrari when my Yugo gets me to my destination?”

asahitoro - Well, the WiiM Ultra is a complete solution and people with many systems are happy to use it for streaming with its integrated DAC. Some also go with an external DAC. 

A WiiM Pro/Plus with an external battery-powered 5v solution will sound better than the Ultra with a good system and fairly good external DAC. The external 5v power is a great option for those WiiM devices. (The WiiM Ultra does not have this option.)

This is somewhat system dependent. For many, the WiiM Ultra as both streamer and DAC will be an easy win. 

You're asking about a device that takes a digital signal and then converts it to analog signal that we can hear.  The digital signal going to the device might be the same, but what comes out is not.  The conversion to analog is where the magic happens and the quality of the DAC and other components makes a difference in how the analog output sounds. 

Short answer is not all streamer/DACs will sound the same and there can be significant improvements in sound quality based on quality of components and level of craftsmanship.

I just received from Germany a Lindemann Woodnote Solo.

Out of the box using as a dac replacement and preamp substitute it really has impressive soundstage..Will try as streamer in the next day or 2.