wght: 100/400/900 | wdth: 151 | slnt: 0 | GRAD: -50/0/50 | opsz: 14

All comments apply to the whole range of grad -50/0/50. There are a few comments which apply to specific grad only - in such cases I made notes to specify the grad value.
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wght: 100 | wdth: 151 | slnt: 0 | GRAD: -50/0/50 | opsz: 14

- нч - appear slightly too narrow.
- breve.cyr.case has perfect width, but is too thin. breve.cyr looks lightly too narrow, too light and its contrast is too high.
- и - diagonal is too strong. It can be slightly thinner in И as well.
- x (component from Latin) - appears too dark.
- ДЛ look slightly wide.
- ф - can be slightly wider.
- э,є - I'm wondering if the bar can be just slightly less long?

I'm wondering about the approach to the wide letterforms:
- шщ are a little narrow in relation to other letterforms. This is an impression I get looking at this exact width 151. But also when I look at шщ across range of widths.
- the opposite situation is ж - it is really wide. Way wider than Ж. Uppercase КЖ seem to be more balanced in terms of proportions with another Cyrillic letterforms to me. Ж left and right counters were narrowed in comparison to К, I would propose similar treatment for LC ж. ШЩ widths seem perfect.
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wght: 400 | wdth: 151 | slnt: 0 | GRAD: -50/0/50 | opsz: 14

- нч - appear slightly too narrow.
- кж - noticed in grad -50 and 0. The top legs are getting significantly thinner while approaching the join. The bottom legs situation is the opposite: the stroke getting darker closer to join. I would try to find a sweet spot.
- м - slightly too dark in grad 50.
- ф - can be slightly wider.
- ПЦ look slightly wide.
- ДЛ as well.
- У - too much weight moved to the left top diagonal, I think it should be thinner. And perhaps right diagonal can be just a little darker?
- К - legs are too thin (especially the top one) in comparison to legs of Latin K or legs of Ж. And stem is too dark.
- З appears just a little dark in grad 0 and 50. I would decrease the weight a bit, but perhaps it could be also a touch wider across whole grad range.
- Яя - Looking at the angle of the legs КЖкж I wonder if legs of Яя can be more diagonal as well.

I'm wondering about the approach to the wide letterforms:
- шщ are a little narrow in relation to other letterforms. This is an impression I get looking at this exact width 151. But also when I look at шщ across range of widths.
- the opposite situation is ж - it is really wide. -к as well. I was comparing it to Latin k but also x,y, and the whole Cyrillic setting. КЖ seem to be more balanced in terms of proportions with another Cyrillic letterforms. ШЩ could be just a touch wider.
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wght: 900 | wdth: 151 | slnt: 0 | GRAD: -50/0/50 | opsz: 14

- нч - can be just a touch wider.
- Ж and ж present significant difference in weight in comparison to each other. Ж sticks out in text - it can be less dark. And ж can be darker on legs. Same for к.
- м, и look a little dark in text. Also ж looks very wide compared to м. ж is too wide, but maybe м could be slightly wider for more even color?
- ПЦ look slightly wide.
- ДЛ look too wide and bright in text. The left diagonal stroke seems to be too light. This is noticeable in whole range of grad, but gets slightly better in grad50. In LC дл the weight is more harmonised across whole grad range.
- в - horizontals are too thin, and letter is too narrow. The issue with thin horizontals repeats quiet a lot. I think mostly in dark weights, but I would recommend to check it in all masters.
- breve.cyr and breve.cyr.case are very light.
- б (grad50) - I'm wondering if the tail could be just a little thinner on top left turn of curve? The width 100 seem to have a little more of such optical compensation.

No files, tests, or entry points are named in the issue. Start by reviewing the Roboto Flex font sources for the listed Cyrillic glyphs across the specified weight, width, grade, and optical-size values; done means the requested width, weight, contrast, and spacing concerns are resolved across the range.

Written by the indexing model from the issue text.