I own a Honda Crf250L and earlier on I have had a weeks ride on Kawasaki Klx250 up in northern Thailand. I have also had a (very) short go on the Himalayan. I did have very positive expectations to the Himalayan as I for example have done longer trips in both Nepal and India and Bhutan on Royal Enfield Bullets and loved it! And in general I have come to like smaller and more lightweight bikes with lower power.
Specifications wise - they all have more or less the same amount of power, the Yammie Wr250R has 31 HP and are by that a bit ahead of the others. The Himalayan has a bit more tourqe as one can expect.
Weight - the Himalayan is around 180 kilograms dry, the japanese 250s are around 130-140 kilos dry. The Honda Crf250 Rally some kilos more. And thats 40-50 kilos lighter than the Himalayan! And knowing how much weight means when youre into the rough stuff the japanese 250s have a huuuuge advantage over the Himalayan weightwise. That said the Himalayan has a low center of gravity and it didnt feel that heavy as the number say when I had a go on it.
Fuel capacity - the 250s comes with gas tanks of 7-8 liters (the Rally has 10,1) and the Himalayan has a 15 liter gas tank. And thats a huge advantage for the Himalayan. The 7-8 liter gas tank on the 250s means their range is 200 kms, maybe a tad more while the Himalayan easily can do 400 kms if you dont twist the throttle all the way. That means if your going for a bit of a distance you will need more fuel capacity on the 250s. And that means bigger gas tanks - which again are quite expensive. You can carry extra fuel in jerrycans, bottles, fuelbladders etc - but imo its all a real PITA out in the real life. A big gas tank is what you need!
The Himalayan also comes with more equipment than the 250s, windscreen, bashplate, and engine bars for protection etc. They also have an option with alu panniers for a bit more bucks.
A Himalayan will also be better for carrying stuff as it is bigger and more sturdy in the first place.
So if you want to set up your bike for a bit of distance riding the Himalayan will be the better out of the box bike. The 250s will need a fair bit of upgrades that easily can be a bit costly.
Reliabilitywise - I think the japanese 250s are still quite far ahead of the Himalayan. They have been around so much longer and by that they much more refined. The first Himalayan model the carburated BS3 got quite harsh reviews. The second edition the EFI BS4 seems to be a bit better. Its not been out that long so that so theres not many users who have done some tens of thousands of kms on them. The most serious problem I have noticed is that the top end have started leaking on at least two occasions, the bolts and the thread to keep the top end at good tourqe to the cylinder seems to be underdimensioned. Its just what I have read, but there has been at least two faults with this. The dutch woman who bought her Himalayan in India aka Itchy Boots (on Youtube) burned her clutch on her bike at quite low milage 10-15 k kms or so? And thats not a good sign.
I have been thinking of upgrading my 250 Honda to a Himalayan but I have come to that conclusion it will not be much of an upgrading. Its so much heavier, it doesnt have more power (a bit more tourqe though) its not as reliable as the Honda, and I have already bought a bigger gas tank to it (if I only had time to mount it...) and if I win the lottery I would consider to upgrade the suspension too on the Honda.
If the Himalayan had 30-35 HP, the weight was 20-30 kilos less and long distance reliability was proven good I would definetively had gone for a Himalayan. But as things are today Im still in for my 250 over the Himalayan.
Here is a Himalayan versus a Honda Crf250 Rally comparison:
https://youtu.be/-i8dYM3MQds