Are Canon crop sensor cameras noticeably mediocre in real world low light situations? Including when you have an all-purpose lens like the 18-150 mounted?
The answer to this may depend on whther you shoot RAW or JPEG, and which software you use to process the images with.
Say you are at a fair at night using such a lens, or you are taking pics of people on an outdoor stage. Is it really going to struggle to focus? The images soft and desaturated compared to competing models?
I used to use 70D, then M5 crop cameras and process with an older version of LR or PS, and I generally limited my ISO to 1600 because I was not happy with the noise beyond that.
I now have R10 and use DxO to process and I have taken shots at ISO 12,800 and got acceptable results. The shots below were with RF 100-400, not RF-S 18-150, and, if anything, the 18-150 may perform slightly better because it is a slightly faster lens (obviously not at 400mm though).
This is not a night shot, but it was late afternoon, in deep shade, and shooting from a safari jeep - at ISO 12,800.
This was also not night time, but in the middle of a tropical cyclone downpour - at ISO 12,800. The camera did struggle a little to achieve eye focus for some shots on the Eastern water Dragon - but that was because I was shooting through a curtain of rain which was confusing it a bit.
Another one in the same downpour - it is a particularly dull area around our pool - in a corner with overhanging trees - at full magnification you can see reasonable detail around the eye and on the scales etc - with acceptable noise levels;
So, in general, no, you should not get mediocre, soft and desaturated results from a current Canon R crop body at high ISO, at least not in my experience. I do not know about the R100 because I think it may use an older sensor.
Specifically which camera were you referring to ?
Those are wonderfully sharp and have nice color. They blow away anything I ever got on my slightly older D3400 with an 18-140 lens. I'm not very good, though. I'm more reliant on the camera to "get it right." But, I never hap sharpness/detail like that, it seems... especially not at higher ISO.
I was referring mainly to the R50, but I know the R50, R10, and I think R7 all use the same sensor and jpeg engine.
AFAIK R10 and R50 share a 24Mp sensor, but the R7 is definitely a completely different 32Mp sensor.
I always shoot RAW, so I really can't comment on the JPEG aspect.
It is, IMO, far better to shoot RAW and do the conversion with DxO. DxO has extremely good lens & camera correction profiles that almost always improve the image sharpness etc. DxO's DeepPrime (and better) noise reduction appears to be industry leading.
Often I find that the process of applying corrections - like those in the images above, are a simple exercise that takes a few seconds. The lens/camera corrections are applied automatically, tick a box for DeepPrime NR if higher ISO was used, and perhaps a light adjustment to highlights & shadows or saturation etc, and convert - done. It is also possible to simply copy the edit settings from one image to any other similar images with a simple copy/paste. I do occasionally use Photshop after DxO but this rare as I find DxO does most of what I want - I usually don't play around much with masks and layers (which is a weakness in DxO), so for most people, DxO probably will cover 99% of their needs.
Unfortunately though, the DeepPrime NR only works with RAW images - it does NOT work with JPEG images - hence the recommendation to shoot RAW. Also, using RAW is generally a FAR better option when shooting in challenging light conditions because it provides the ability (in any RAW editor) to have far more latitude with making exposure or white balance adjustments.
As regards the R50 - I don't have one, but be aware that there is a pretty long laundry list of differences between it and R10, many of which are not detailed clearly in the spec sheets, or even most online reviews. I, and others, have posted (probably) most of those differences in a few threads in these forums. Some are quite significant, like automatic sensor cleaning, and others are less obvious, like R50 not being compatible with the much faster UHS-II SD cards, which is a double-whammy because R50 has a significantly smaller buffer than R10 (which is already a lot smaller than the likes of R8).
So my advice is to do some research to become familiar with those differences, and consider how they may impact you now and in the future (buy once and get it right, rather than buy cheap, buy twice).
If you only ever plan to shoot "happy snaps", mostly for social media (or have another much larger more capable camera like some R50 owners here - which I don't think describes your situation), then R50 may be perfectly adequate. However, if you intend getting a bit more serious about photography and think you may grow into it going forward, then R10 is probably a far better option.
As regards a FF camera, like the RP mentioned above, yes, FF is generally better in low light (high ISO) than a crop sensor. I have R10 and R8, and R8 was the "successor" to RP. R8 is better in low light than R10, but I do think that DxO has certainly helped to narrow the gap with high ISO performance.
Note that R8 body is quite a bit more costly than R10, and much more than R50, and the FF lenses are generally more costly as well, so while going FF certainly has advantages, it does have downsides. FF loses the 1.6x crop factor, so a 100-400 lens is 100-400mm, not a FF equivalent FoV of 160-640mm like if it was used on a crop camera.
This is a shot from R8 at ISO 25,600, processed with DxO. Best viewed at Original size and magnified.
