If SMR is well designed firmware / cache wise, it allows large capacity products to hit the market faster and with equivalent or slightly better performance under light loads--since these products usually have 128MB+ cache and (hopefully) media caching as well.
The easy way to determine if a SMR product will fall flat under heavier loads is to run a normal (1GB) Crystal Diskmark test--but with the write cache disabled. Modern Seagate 2.5" SMR products I've tested (which have been on the market for years) still continue at a reasonable speed, but WD 2.5" products such as their WD10SPZX run horribly. Even its sequential read performance drops severely, so its cache algorithms are very badly designed for handling write saturation.
I haven't tested a newer gen WD Red, but after running these tests I'm not surprised at all. Furthermore, using SMR on a drive in NAS/RAID is simply a bad idea in general because of the expectation of sustained write load during a rebuild scenario. Until you are absolutely sure that your cache algorithms (even with write caching disabled) can deliver reasonable QoS / latency / throughput during that type of load, this type of product should not be marketed.
I'm attaching the WD10SPZX (5.4k rpm) results with its write cache on and then off, and then two SMR Seagates (ST1000LM035-5.4k and ST500LM034-7.2k) with write caching disabled for perspective:
WD10SPZX firmware 201a02, write cache on:
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CrystalDiskMark 3.0.4 x64 (C) 2007-2015 hiyohiyo
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* MB/s = 1,000,000 byte/s [SATA/300 = 300,000,000 byte/s]
Sequential Read : 128.392 MB/s
Sequential Write : 124.431 MB/s
Random Read 512KB : 43.944 MB/s
Random Write 512KB : 109.451 MB/s
Random Read 4KB (QD=1) : 0.504 MB/s [ 123.0 IOPS]
Random Write 4KB (QD=1) : 9.036 MB/s [ 2206.0 IOPS]
Random Read 4KB (QD=32) : 0.611 MB/s [ 149.2 IOPS]
Random Write 4KB (QD=32) : 1.272 MB/s [ 310.5 IOPS]
Test : 1000 MB [D: 0.0% (0.1/931.5 GB)] (x5)
Date : 2020/05/03 14:54:46
OS : Windows 10 Professional [10.0 Build 18363] (x64)
WD, write cache off:
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CrystalDiskMark 3.0.4 x64 (C) 2007-2015 hiyohiyo
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* MB/s = 1,000,000 byte/s [SATA/300 = 300,000,000 byte/s]
Sequential Read : 48.871 MB/s
Sequential Write : 11.078 MB/s
Random Read 512KB : 17.899 MB/s
Random Write 512KB : 20.634 MB/s
Random Read 4KB (QD=1) : 0.457 MB/s [ 111.5 IOPS]
Random Write 4KB (QD=1) : 0.161 MB/s [ 39.4 IOPS]
Random Read 4KB (QD=32) : 0.966 MB/s [ 235.8 IOPS]
Random Write 4KB (QD=32) : 0.162 MB/s [ 39.5 IOPS]
Test : 1000 MB [D: 0.0% (0.1/931.5 GB)] (x5)
Date : 2020/05/03 14:34:40
OS : Windows 10 Professional [10.0 Build 18363] (x64)
ST1000LM035 firmware RSM8, wc disabled:
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CrystalDiskMark 3.0.4 x64 (C) 2007-2015 hiyohiyo
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* MB/s = 1,000,000 byte/s [SATA/300 = 300,000,000 byte/s]
Sequential Read : 146.062 MB/s
Sequential Write : 16.305 MB/s
Random Read 512KB : 47.639 MB/s
Random Write 512KB : 9.714 MB/s
Random Read 4KB (QD=1) : 0.574 MB/s [ 140.1 IOPS]
Random Write 4KB (QD=1) : 0.212 MB/s [ 51.8 IOPS]
Random Read 4KB (QD=32) : 1.328 MB/s [ 324.3 IOPS]
Random Write 4KB (QD=32) : 1.184 MB/s [ 289.1 IOPS]
Test : 1000 MB [D: 0.0% (0.1/931.5 GB)] (x5)
Date : 2020/05/03 18:19:45
OS : Windows 10 Professional [10.0 Build 18363] (x64)
ST500LM034 firmware SDM2, wc disabled:
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CrystalDiskMark 3.0.4 x64 (C) 2007-2015 hiyohiyo
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* MB/s = 1,000,000 byte/s [SATA/300 = 300,000,000 byte/s]
Sequential Read : 172.861 MB/s
Sequential Write : 22.112 MB/s
Random Read 512KB : 56.960 MB/s
Random Write 512KB : 12.890 MB/s
Random Read 4KB (QD=1) : 0.682 MB/s [ 166.5 IOPS]
Random Write 4KB (QD=1) : 0.273 MB/s [ 66.6 IOPS]
Random Read 4KB (QD=32) : 1.453 MB/s [ 354.7 IOPS]
Random Write 4KB (QD=32) : 1.242 MB/s [ 303.2 IOPS]
Test : 1000 MB [D: 0.0% (0.1/465.8 GB)] (x5)
Date : 2020/05/03 15:27:18
OS : Windows 10 Professional [10.0 Build 18363] (x64)
A PCMark benchmark on the ST1000LM035 with its write cache disabled dropped to ~50% of its write cache enabled score--which is certainly acceptable performance for SMR in its "worst case" write performance state. With its write cache enabled, its PCMark score was slightly higher than an older generation WD MZ500S 7.2k 2.5" CMR drive with 32MB cache.
I would describe Seagate's modern SMR implementation as "done right" for average client use / desktop scenarios, but I still wouldn't want these compromises in a NAS product.